I know you said "establish himself elsewhere" on purpose, but Miazga had done pretty well for himself. Playing in Europe, getting paid a Prem salary and still on the NT radar is not bad at all, even if he's had to go on a number of loans away from Chelsea.
Everything is relative. If in five years Omir Fernandez is a rotation player for Wigan we'd champion it as an incredible success. If Tyler Adams is doing the same we'd be wondering what the hell went wrong for him.
I'd say that Miazga has been a moderate failure based on expectations. I think most people expected him to be a no-doubt USMNT starter, or at least close to it, by now. Surely a regular starter for a team in the top flight of a good European league.
More to my point, I don't see how staying another year or so at RBNY would have put him in a worse position to develop than what has instead transpired.