Your thinking more of the NASL. USL was really built around accepting its place in the system and thriving there. They have some good, well-run clubs. I dont think they have a significant anti-MLS supporters base, mostly just a "this is our local club" type of following, then rounded out with serious fans of MLS who watch it because it's more soccer.I am genuinely asking, is there a fan base for the USL? I feel like the whole schtick is that they aren't MLS and don't have a salary cap. But what little I know of the league is that it currently holds many MLS 2 franchises (which makes it really funny how they are against MLS but house all the leagues reserve team) with a few free standing clubs. Therefore it gives off a minor league look. So how are they even trying to compete with MLS/be the first division in the US if it has all of the MLS 2nd teams? It seems to me that if soccer wasn't hipster enough in the US (and Canada) the USL has the hipsters of the hipsters following their league, so to speak. Seems kind of funny if you ask me. Plus how can you talk of Pro-Rel if the 2nd teams can't move up to the first division (MLS)? Would you make a league below USL and bleed money as the second division with USL being the first division? Seems really odd to me.
USL was struggling to gain footing after the TOA clubs split to form the NASL. After a few years, MLS was looking to partner with NASL, then the only division 2, to accept its reserve teams and creating a smooth transfer/loan process to other teams. NASL was allegedly close to accepting the partnership, until the Cosmos owners came along and forced out the commissioner and pushed the league away from that strategy. USL took the offer and was able to build out a full league around it while NASL went to oblivion.
Now, USL has well established independent clubs nationwide and the MLS partnership is not vital anymore.