I'm not sure it could've lasted much longer, even in a vacuum. I can't think of a single ethnically-diverse land-based empire that lasted for any great length of time before the modern period.

...though I'm sure several are about to be listed.

This also goes to why the England/France debate misses the point, namely that they're still England/France because the people who live there are English and French. The question isn't maintenance of nationhood, but maintenance of empire.