First point worth making is that I don't think it matters what the Hawks fans call themselves, nor will it have any impact on their fandom.
Three other thoughts:
1.
RolandDeschain":1aobwttx said:
Seeing as how the 12th Man phrase was coined half a century before Texas A&M started using it in another part of the country altogether, I'm guessing it wouldn't be hard to strip them of the trademark they shouldn't have been given in the first place. Prior art would screw Texas A&M over in a big way; it's in their best interest not to poke the hornet's nest.
Perhaps, but part of the licensing agreement Seattle signed with A&M was public acknowledgement and legal agreement that A&M is the rightful owner of the trademark. This was written in precisely so that the Seahawks couldn't decide that they wanted to stop paying every year to use the trademark and then try to argue that it belonged to common usage. It was a smart move on A&M's part, and one the Seahawks agreed to.
2.
With regards to just switching to "twelves", "12th fan", etc., etc., these types of variation are protected under trademark law, and are generally a no go (e.g. the joke of the golden arches of "McDowell's" in COMING TO AMERICA -- it's a joke because in the real world it would be a clear infringement on McDonalds' trademark.
3.
Neither of these things means that a judge couldn't decide to rule that "12th Man" has
become a term in generic use (arguing that the trademark should never have been granted in the first place is a much harder row to hoe, see #1). Not sure how applicable that is to this case because we don't see the proliferation of "12th man" usage across a bunch of different NFL and college teams in the last 35 years (which is what you'd have to show), but stranger things have happened.
The recent example is that after a ten year lawsuit the variant on yoga called "pilates" was found to have become a term that was generically used and could no longer be trademarked. It was only noteworthy because pilates is named after its invenor, Joseph Pilates, and his family lost the right to control the marketing of their own last name. So, yeah, stranger things have happened. (although MANY other general use terms -- dumpster, band-aid, windbreaker, jacuzzi, bubble wrap, jet ski, popsicle, kleenex, etc., etc., etc., -- are still trademarked).