The issue is that there’s no generally correct behavior. Should a database under memory pressure stay up at all costs even if it becomes unusably slow (by e.g. nuking 99% of the buffer cache)? Or should it crash/failover hard with a likelihood of potential recovery afterwards, even if it technically could have stayed up? Something in between?

There’s no correct-in-general answers to those questions. This is a hard problem due to context dependence; that’s why there are so many knobs.