I think it's only theoretically impossible. As a practical matter, I don't think it's much of a problem. I believe we could work with the minority of contributors who believe their content cannot be migrated in a way that is satisfactory to them.
You mean "the minority of contributors who believe their content cannot be migrated in a way that is satisfactory to them *and find out what we're doing*". I think you're mainly relying on people not noticing, which doesn't seem very moral to me.
Clauses like '...or later versions.' are invalid in German jurisdiction and only the rest of the license applies. [Delphine makes the same point about French law.]
... but I think as a practical matter this is not a problem, because you'd have to find a copyright-holding Wikipedian in Germany who (a) doesn't want to relicense, and (b) would prefer to spend money on litigation rather than simply withdraw his or her contributions (from Wikipedia, not from any other venue that is using the content consistent with older GFDL versions) . And even if you could find someone for whom both (a) and (b) are true, the fact that WMF would likely withdraw the contributions anyway if litigation were even hinted at would go a long way towards building a legal defense for WMF in such a case.
Withdrawing contributions would be a nightmare, as I explained in a previous email. That aside, if we agree that we need their permission to relicense, then surely it needs to be opt-in, not opt-out? "It's only illegal if someone sues you" may work in practice, but it doesn't seem like a very nice way to behave.