> I understand the many reasons why that's not possible

Isn't the only reason that the UI effectively freeze under high memory pressure? If the processing involved in the core parts of the UI could be prioritized there's no reason for it not to work?

I don't understand why we can't have (?) a way of saying "the pages of these few processes are holy"? (yes, it would still be slow allocating new pages, but it should be possible to write a small core part such that it doesn't need to constantly allocate memory?)