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.
