In the end, all that really matters is that the people who own the copyright to Wikipedia aren't interested in going through the hassle and expense of suing the WMF.
Wikipedia has made plenty of enemies over the years, some of them fairly wealthy. I think there is a good chance of someone deciding to cause trouble if they think they can win. It only takes one.
It only takes one to do what? If one of Wikipedia's enemies wants to spend lots of money causing trouble, there are plenty of issues they could raise right now, and, as I noted before, there's a good chance they could win (though the monetary award in any case would probably be negligible).
And if any of them actually were, the WMF would probably be willing to remove their contributions way before it got to that point anyway.
That's easier said than done. It would probably end up requiring the deletion of any article they contributed to, unless someone's willing to take the time to go through each article and just delete the parts they specifically contributed to.
In order to enter into a copyright lawsuit in the United States, you first have to register your copyrighted work. So anyone serious about starting such a lawsuit has to first figure out what it is s/he claims copyright over, and moreover who the copyright holders are. If they claim copyright over the entire article of which they are only one contributor, then they'll have to also note all the other joint authors on that application. And doing that opens up the defense that any joint author can independently grant a license on the work. If, on the other hand, they decide to take the position that they are a sole author of a particular sentence or paragraph, then that is the sentence or paragraph which can be removed. Multiply by all the sentences or paragraphs they authored, and it will be a lot of work, but considering the fact that "it can be argued pretty strongly that Wikipedia is not currently complying with GFDL fully" it's work that has to eventually be done if someone decides to sue, *regardless* of whether or not the license is changed.
And, of course, in the case of the WMF, there's that other step of issuing a DMCA takedown notice. A successful lawsuit would be difficult. Possible, in my opinion, but difficult.