As someone who's worked with PostgreSQL for a decade now, I've very definitely seen cases where memory bandwidth is the bottleneck, not disk bandwidth. I'm pretty sure the IO subsystem would still be a narrower pipe than main memory, even if you stuck Optane devices in every available connector at the same time and striped across them.

EDIT: Don't mistake me, I'm very excited about the potential of Optane devices in the workloads my databases handle (though, since my postgres machines are all in the cloud right now, that remains a purely theoretical question). It's just not a panacea.

Then again, nothing is.