There's nothing stopping someone from adapting an emulator to Stadia's rendering framework, although I'm not sure Google would approve the result. I agree with you that Google acquihiring (for example) Good Old Games might have been cool, but IMHO Google is pitching the other end of the performance spectrum: play AAA games on a Chromebook.

If they had done both then they could have released GOG titles as filler between AAA releases and that would have been something.