Here is link giving a bit of details that Buddha preached about the decline of Buddhism after parinibbana.
I share with you the following passage taken from the page of the above link
The Buddha said that after His parinibbaana, only for 1,000 years will the monks be able to acquire the analytical knowledge (Patisambhida), or what we know as Arahanthship or deep knowledge. Then, as time goes on, they will only be able to attain Anaagami (Never return). And then only Sakadagami (Once return), and finally, only Sotapanna (Stream enterer). With the death of the last disciple who has attained Sotapanna, the attainments will disappear.
Here's my issue with the above. From what I understand is that after a certain period of time after parinibbana, the maximum attainment that one could achieve reduces from arahant to anagami and so on. But this is illogical because if a person attain anaagaami he/she will only have at most 1 life where he/she will become an arahant which makes the above statement false, unless it means that the person will be born as a deva maybe having a very long lifetime until the next Buddha. This is confusing.