I would deem this a non-issue. We have already seen examples of studios loaning out rights if need be. Universal would certainly be willing to loan out rights of Namor to Fox or Disney in exchange for a hefty fee. Obviously Feige is cheap as hell so don't expect Namor in the MCU anytime soon, but Fox already acquired Kingpin from Sony in the past, unless Avi Arad was just fabricated the whole loaning Kingpin to Fox story. In this case, as long as Fox or Disney are willing to put up some money, Namor will be featured. Under no circumstances will he get a standalone movie, the exception being if Aquaman gets greenlit and is a big hit, maybe Universal would try to replicate that, or perhaps try to beat Warners to the punch.