I understand that you might like to have both [features and stability] but I think that's just fundamentally impossible -- big new features just tend to require deep, invasive changes.

I think we could encourage that, I don't think it's really something
you can make a hard requirement. spatches just don't work well for all
kinds of API changes. Starting this as a sort of "experiment" like you
suggested to see how it goes sounds like a good idea.