Frankfurt is no London and no Paris, this is clear. However, that's also related to the way in which Germany as a whole functions. The individual cities aren't all that big, but they are clustered in metropolitan areas.
If you live in the vicinity of Frankfurt, you can just drive to Heidelberg for a Saturday night out (about an hour). Or, if you're more interested in food, theater or classical music, you have a large selection from Mainz to Aschaffenburg, all of which is easily reachable from Frankfurt.
Equally, the pool of support staff you can draw from isn't recruited from the minuscule population of Frankfurt (700k), but from the whole metro region (5.5 million). I myself live at the outermost boundary of the metro region and know many people who commute to Frankfurt.
The second point is that the relatively small size of the city of Frankfurt is an asset in one critical regard: Frankfurt is first and foremost a banking city (and secondly an airport city). If banks want to exert some influence over policy there, they have all the leverage they could want. This is not the case in Paris.
Personally, I'd be surprised if there was anything other than a somewhat even split of relocations between Frankfurt and Paris. And I'm fairly certain that the TGV service between Frankfurt and Paris will also be extended beyond what exists today.