You're assuming some things in your reasoning. One, that theocracy will "collapse finally". There's historical precedent to say that it may well not collapse at all. It may well be that a theocratic authoritarian quasi-totalitarian government is quite stable and "sustainable". After all, all it can promise is a life of material misery but spiritual wonders (and if you yourself are not experiencing spiritual wonders, you're probably an atheist and should be executed anyway). Such a "state" is perfectly sustainable, especially if the people having the most children are the least secularly schooled. I'm not saying this is the case, I'm saying you are assuming the opposite is true, without much justification.
Two, you're assuming that Ataturk's scheme was to push "too hard and too far and too fast".
Three, you're assuming that the problem was this authoritarian secularist push, and not any other factors coming in. Like, say, demographics. Or external factors, like, say, the Iraq war, the Syrian war, ISIS, the refugee crisis, etc. Or like, say, the coming to prominence of a singular character that is behaving quite like a Sultan and able to destroy any democratic institutions and culture that Turkey still had.
That kind of authoritarian asshole is probably even more "unsustainable" than a coup (and I think you agree with me here), but given how Putin has survived and thrived for so many years now (and given the alternative that is Syria), it's probably not out of his reach to be able to destroy the entire democratic edifice and build his own dictatorship before he gets ousted or killed.
So I don't see anywhere here a prospect for "sustainability", unless we are talking about the Afghanistization of the country (Turkey doesn't have, contrary to Iran, oil or gas - at least on relevant levels). Thus the choice isn't really between "sustainability" and "chaos". It's between religious totalitarianism and authoritarian secularism. I'm not really enamored to any of those, but without hesitation, I'll pick the latter.