But I'm sure even in ancient times it was not the WHOLE nations that were on the move. There were some individuals (or perhaps even groups) that chose to stay (see Avari "The Silmarillion") or turned back at an early stage.
Look, you got me - there's a whole can of worms on cultural identity and group membership that I wanted to avoid opening up, so I used a 'quick and easy' shorthand.

A more precise and sociologically-neutral way to put it would be:

Whole communities, or large parts of them, no longer travel cohesively (i.e. constituting a sociopolitical unit) from an origination point to settle, permanently or otherwise, at some other point. A community here can be supralocal, e.g. in the sense of the Nordic settlers of Iceland.

Actually, from that point of view Israel/Soviet Jewry is an interesting case, since with Israel you had many local communities loosely-connected by shared traditions and a nascent Zionism converging on one point in order to undertake a project of forming a new "nation". Of course, if you're a hard-core Zionist that analysis would be tendentious, but really a broader Jewish identity existed only in a relatively-limited number of intellectuals and political activists, even as recently as a century ago. Similar with the Vietnamese case.