You don't have terrible PC graphics because of the much lower end PCs that can play this game, why would you have them because of consoles all of a sudden?
Thats not how this works.
because of standardization i would say, although i'm not very informed on the subject you're right.
but if cross play is actually working it could means that the development of all version will be unified, thus reducing the overall quality to match the console version. (like shadows, textures, fog...)
but i'm probably wrong