It's quite a big leap from a single isolated electron to a system with hundreds of them (not to mention QCD interactions). While you are right that there is no

It's quite a big leap from a single isolated electron to a system with hundreds of them (not to mention QCD interactions). While you are right that there is no a priori reason to dismiss that, it's not quite a bagged thing. Especially considering that we are missing some things for sure (neutrino mass, ...)