^^ Straczynski faced the same problem of network interference when he was producing Crusade for TNT. After initial appeasement he grew weary of their meddling and simply refused to take their 'direction' on board. He stuck to his guns and effectively forced TNT to cancel the show. He didn't need to leave or violate his contract with them, and as such gained more kudos from B5 fandom for not allowing the direction of the show be dictated by network suits. It also hasn't prevented him from finding more work in both the television and comicbook fields.
Why couldn't Berman/Braga do the same? They didn't need to leave, just refuse to be dictated to and then force them to either replace you, or cancel the show. Either way, you're free from your contract without the threat of legal action.
And let's face it, worst case scenario, they're blackballed by UPN. Whoopy fcuking doo......That would be a real career killer.....Not.
Moore left, and after having a creative hand in Roswell and Carnivale, runs nuBSG. Echevarria and Behr left and came up with The 4400. Piller left and gave us The Dead Zone. There's life after Star Trek and the above have been able to carve their niche. Perhaps Berman/Braga realised that they simply weren't as creative as them and therefore couldn't cut the apron strings to free themselves from Star Trek. Lord knows, if Threshold is anything to go by, that's a fairly safe bet....