Although "Dhamma" is in the title, the text of the question seems to be asking instead about the Discipline:
If the Sangha has become a group of householder wearing robes, should the Noble Sangha let go of their heritage? ...
Would the Buddha allow monks to give up Sangha heritage in that case?
I read The Broken Buddha so it's not completely new to me to hear of someone saying something like, "the leaders of the Sasana are all but householders".
I wouldn't want to say that, and I couldn't say it about anyone (because I don't know them), but I suppose that what's written in the book has some truth in it, even if only that it's as-witnessed.
Similarly I read a little of the "austere Forest Tradition" -- specifically as contrasted with the living and so on in towns.
I don't have a specific answer for you -- I think there are several:
- You might want to find a monastery which you approve of -- a bikkhu, a group of bikkhus, who you approve of. People's biographies, monks and nuns too, often include their visiting and studying in several places, studying under several teachers.
- It's common to disrobe -- not that I'm recommending that, but perhaps that's what you meant by "give up Sangha heritage". Perhaps some people who do that give up parts of the the Discipline, but not the Dhamma.
- Perhaps consider being less critical of others -- I assume you are critical of others, "You shouldn't practice/live that way", "You don't have a proper teacher", "Your motives aren't proper", etc. To pick an example almost at random, there's this Wikipedia article about Ajahn Sumedho -- which describes him as "engaging and witty communication style", and pictures him and the monk who he's talking with laughing together. Then again the biography of Ajahn Chah says, "He sometimes initiated long and seemingly pointless work projects, in order to frustrate their attachment to tranquility" -- possibly you have some attachment to the way things ought to be, how other people ought to behave.
Is that, the degeneration, actually the reason why such as householder movements became that popular?
How popular is "that popular", what "householder movements"? Perhaps what you're talking about is an "observation bias" i.e. if you observe householder movements then that's what you see; and if you e.g. read the internet less then you wouldn't see them so much -- but I don't know what you're talking about. You see different things than I do, and perhaps you see them differently -- the all.
There are articles in the Appendix which say e.g. (in 1931) that ...
Already prominent laymen in Burma, Siam, Ceylon, and elsewhere, view with misgivings the present state of affairs and know that sooner or later some alterations will have to be made. Nearly everyone sees signs of decay in the Order, that Order that has continued for 2500 years, but today there are new conditions and forces in the world and unless something radical is done this decay will increase until either the Sangha dies out, or becomes a dead letter, the refuge of the ignorant and unworthy.
It isn't only monks of Western origin who say this. Anyway you might want to read The Broken Buddha assuming you haven't already. I think it talks about issues (i.e. problems within Sangha) which you question here, and quote various people who suggest solutions, and maybe you can find in it some people or organisations who you might concur with.
I don't know that I can answer this question, though, since I don't know what you mean by "householder movements".
I suppose that everyone (householders too) must practice on their own (as "islands"), AND that the Sangha is, has been, will be important.
Should the Noble Sangha let them follow their inclinations which brings not only them long time suffering but for many?
I don't know that it's possible to prevent people from doing what's harmful -- if you knew how to, perhaps you could help drug addicts? -- and if you can't prevent them then I don't see how it's a question of "letting them".
If someone only sees you as getting into arguments, and scolding people, etc., I suppose that's not effective.
Resting simply in "Beings are heirs of their kamma..." or still share as much as compassion as possible to keep those able away from doing really grave wrong doings?
I think the theory is that it isn't one OR the other (i.e. compassion OR equanimity) -- instead it's both (i.e. "and") -- or rather, all four (mudita and metta too).
Also you practice one whenever you can't practice another -- or perhaps one is an antidote to an excess of the other.