That's not a solution to OS instability.

Reliably saving state in the face of sudden total failure is both very tricky and app-specific. Just saving state changes automatically won't do it -- partial writes of complex state are likely to be inconsistent without luck or careful design and QA controls (tests, testing, on-going controls to ensure nothing new operates or relies on anything outside the safe state-saving mechanism).

It makes a lot more sense to put the effort into making the OS continue as well as it can, vs requiring every app to harden itself against sudden total failures.

No, this is why kernels prioritizing not crashing is important. Applications saving their work is a nice extra.