Considering how many people are using cases and even wallet cases, probably very many people. The case/wallet can be attached to the phone as a replacement to the normal backplate, making it much less bulky, just as Samsung did forever ago.

Yet nobody is making high-end plastic phones anymore. Because nobody would buy them (just like no nobody cared about replaceable batteries so all companies stopped making them).

I don’t think that the case situation is necessarily rational but people generally seem to prefer more fancy/expensive/better looking phones these days.

The pixel a series is made of plastic too, today. I'm not seeing anybody bothered by that.

Umm..late comment, but people are still buying smartphones made out of plastic, even recycled plastic these days.