then how can we be even sure about enlightenment

I think there are schools of Buddhism which say that enlightenment becomes less possible, as the centuries go by and the Buddha's doctrine becomes distorted.

There are also schools which say that enlightenment is already attained (by the Buddha), therefore enlightenment is certain.

Or that all saṅkhāras (conditioned things) are impermanent, but that some dhammas (especially nirvana) are unconditioned (and not-self), and "timeless" (or immediate).

People also often say that it (the path to enlightenment) is something you should experience for yourself, i.e. have direct experience of.

if sansara can change

I think that sansara is expected to change: that it's considered "impermanent" in Buddhism.

What seems more-or-less constant about it is a cycle of death and life (perhaps you you'll agree that death and life still exist), and suffering (oceans of tears), and causes of suffering, etc., as described in Dharma.

what is to say that some day all beings that achieved nibbana will not again be reborn?

I think that some schools of Buddhism assert that some relatively-enlightened beings are reborn (see for example "Bodhisatvas" and "tulkus").

But perhaps the statement "beings that achieved nirvana may be reborn" is a nonsense phrase, for example because it assumes that "the same being" may be "reborn" ... whereas, sansara and impermanence posit that anything "reborn" is necessarily a different being.

Also, enlightenment and "being" might be mutually contradictory. It reminds me of this Vajira Sutta:

Why now do you assume 'a being'?
Mara, have you grasped a view?
This is a heap of sheer constructions:
Here no being is found.

I think that's connected with for example this discussion of 'trackless'.

Beware too that this is (or is nearly) a topic for which it's difficult to get sensible answers: the unanswered questions include questions about whether the Tathagata exists after death, and the four imponderables includes

The Buddha-range of the Buddhas [i.e., the range of powers a Buddha develops as a result of becoming a Buddha]

The warning is that speculation on these topics may be counter-productive -- not the right way.

then how can we be even sure about enlightenment

I think there are schools of Buddhism which say that enlightenment becomes less possible, as the centuries go by and the Buddha's doctrine becomes distorted.

There are also schools which say that enlightenment is already attained (by the Buddha), therefore enlightenment is certain.

Or that all saṅkhāras (conditioned things) are impermanent, but that some dhammas (especially nirvana) are unconditioned (and not-self), and "timeless" (or immediate).

People also often say that it (the path to enlightenment) is something you should experience for yourself, i.e. have direct experience of.

if sansara can change

I think that sansara is expected to change: that it's considered "impermanent" in Buddhism.

What seems more-or-less constant about it is a cycle of death and life (perhaps you you'll agree that death and life still exist), and suffering (oceans of tears), and causes of suffering, etc., as described in Dharma.

what is to say that some day all beings that achieved nibbana will not again be reborn?

I think that some schools of Buddhism assert that some relatively-enlightened beings are reborn (see for example "Bodhisatvas" and "tulkus").

But perhaps the statement "beings that achieved nirvana may be reborn" is a nonsense phrase, for example because it assumes that "the same being" may be "reborn" ... whereas, sansara and impermanence posit that anything "reborn" is necessarily a different being.

Also, enlightenment and "being" might be mutually contradictory. It reminds me of this Vajira Sutta:

Why now do you assume 'a being'?
Mara, have you grasped a view?
This is a heap of sheer constructions:
Here no being is found.

I think that's connected with for example this discussion of 'trackless'.

Beware too that this is (or is nearly) a topic for which it's difficult to get sensible answers: the unanswered questions include questions about whether the Tathagata exists after death, and the four imponderables includes

The Buddha-range of the Buddhas [i.e., the range of powers a Buddha develops as a result of becoming a Buddha]

The warning is that speculation on these topics may be counter-productive -- not the right way.

then how can we be even sure about enlightenment

I think there are schools of Buddhism which say that enlightenment becomes less possible, as the centuries go by and the Buddha's doctrine becomes distorted.

There are also schools which say that enlightenment is already attained (by the Buddha), therefore enlightenment is certain.

Or that all saṅkhāras (conditioned things) are impermanent, but that some dhammas (especially nirvana) are unconditioned (and not-self), and "timeless" (or immediate).

People also often say that it (the path to enlightenment) is something you should experience for yourself, i.e. have direct experience of.

if sansara can change

I think that sansara is expected to change: that it's considered "impermanent" in Buddhism.

What seems more-or-less constant about it is a cycle of death and life (perhaps you you'll agree that death and life still exist), and suffering (oceans of tears), and causes of suffering, etc., as described in Dharma.

what is to say that some day all beings that achieved nibbana will not again be reborn?

I think that some schools of Buddhism assert that some relatively-enlightened beings are reborn (see for example "Bodhisatvas" and "tulkus").

But perhaps the statement "beings that achieved nirvana may be reborn" is a nonsense phrase, for example because it assumes that "the same being" may be "reborn" ... whereas, sansara and impermanence posit that anything "reborn" is necessarily a different being.

Also, enlightenment and "being" might be mutually contradictory. It reminds me of this:

Why now do you assume 'a being'?
Mara, have you grasped a view?
This is a heap of sheer constructions:
Here no being is found.

I think that's connected with for example this discussion of 'trackless'.

Beware too that this is (or is nearly) a topic for which it's difficult to get sensible answers: the unanswered questions include questions about whether the Tathagata exists after death, and the four imponderables includes

The Buddha-range of the Buddhas [i.e., the range of powers a Buddha develops as a result of becoming a Buddha]

The warning is that speculation on these topics may be counter-productive: not the right way.

then how can we be even sure about enlightenment

I think there are schools of Buddhism which say that enlightenment becomes less possible, as the centuries go by and the Buddha's doctrine becomes distorted.

There are also schools which say that enlightenment is already attained (by the Buddha), therefore enlightenment is certain.

Or that all saṅkhāras (conditioned things) are impermanent, but that some dhammas (especially nirvana) are unconditioned (and not-self), and "timeless" (or immediate).

People also often say that it (the path to enlightenment) is something you should experience for yourself, i.e. have direct experience of.

if sansara can change

I think that sansara is expected to change: that it's considered "impermanent" in Buddhism.

What seems more-or-less constant about it is a cycle of death and life (perhaps you you'll agree that death and life still exist), and suffering (oceans of tears), and causes of suffering, etc., as described in Dharma.

what is to say that some day all beings that achieved nibbana will not again be reborn?

I think that some schools of Buddhism assert that some relatively-enlightened beings are reborn (see for example "Bodhisatvas" and "tulkus").

But perhaps the statement "beings that achieved nirvana may be reborn" is a nonsense phrase, for example because it assumes that "the same being" may be "reborn" ... whereas, sansara and impermanence posit that anything "reborn" is necessarily a different being.

Also, enlightenment and "being" might be mutually contradictory. It reminds me of this Vajira Sutta:

Why now do you assume 'a being'?
Mara, have you grasped a view?
This is a heap of sheer constructions:
Here no being is found.

I think that's connected with for example this discussion of 'trackless'.

Beware too that this is (or is nearly) a topic for which it's difficult to get sensible answers: the unanswered questions include questions about whether the Tathagata exists after death, and the four imponderables includes

The Buddha-range of the Buddhas [i.e., the range of powers a Buddha develops as a result of becoming a Buddha]

The warning is that speculation on these topics may be counter-productive -- not the right way.