> I am no lawyer, but wouldn't be enough if the derivative builds also released their
> code under the new license (with updated header files?
Yes, that's literally all that needs to happen. There haven't been nearly as many external contributors to projects like MAMEUI or MAMEUIFX as there have been to MAME, so it should be an absolute walk in the park to take care of. If e.g. Mamesick wanted to keep MAMEUIFX going, it would be trivial to do so, and the fact that he's instead using this as an opportunity to stir up anger and resentment amongst his doe-eyed sycophants is really telling.
The fact of the matter is that people like Mamesick have been abrasive towards the MAME team at best and downright abusive at worst over the past decade, and have made it abundantly clear that they would see everything MAME has accomplished over the past 19 years razed to the ground if it meant having one more game become "playable" for the rest of the l33t fr33 g4m3z!11`! crowd who don't give a shit about accurate emulation. As usual, he's acting like a spoiled child who isn't getting his way, and it's a shame that people like Big Karnak and crew are being taken in by his vitriol.
It seems that the main genesis of this rift was 2-3 weeks ago, when the MAME team started cleaning up legacy code and removing old, less-than-maintainable bits. It's at this point where Mamesick's sense of unwarranted self-importance, and that of the rest of the people trying to keep MAME running on XP and earlier, really started to show. According to these people, we are literally Hitler for caring more about getting MAME running on modern devices than we are for catering to people using PCs older than some of the people who post on Tumblr. It's this sense of unwarranted self-importance that causes these people not to realize that in the long run, it's going to be MAME running on these more modern devices that attracts new developers and new talent to the team, not MAME running on a 486 that was fished out of a dumpster or traded for a fucking dime bag.
It's really unfortunate that the "free games" crew is under the mistaken impression that they're the ones keeping MAME alive, rather than the progressive modernization of both MAME's code and its policies over the past year, so let me be very clear: Either you're aboard the MAME progress train, or you're standing in front of it - in which case, get the fuck out of the way before you get run over.