> So might Lattner if he were starting from scratch.
Some, yes, but not much as he has been asked about it before.
The leadership has no problem completely replacing or removing APIs so stability is not a problem. Users simply have to bear the pain. Pain does not seem to be a concern of the project. And that's fine. I completely understand why they choose to operate that way. I wouldn't want to be locked into a set of APIs either.
Certainly they did get a lot right. As for real-world impact, I suspect that something like MSVC is pretty modular and that Microsoft uses that to its advantage in Visual Studio. Of course we can't prove that.