If you can afford to do that, it's nice to do it indeed. It can get messy and hairy quite quickly though.

It seems to me a lot of it boils down to "who gets impacted". If I have a bug in my kernel and a lot of apps are somehow relying on it, I need to be mindful of that and perhaps swallow the purity of the contract in order to provide a pragmatic fix.

Sure, it's not a strategy without costs and Microsoft goes to an extreme. But there's probably a middle way in there; if you're aware that tons of programs now depend on the buggy behavior you probably want to deal with it. But if you're working on Windows or the Linux kernel the tolerance for breaks is probably much lower.