The 2k series did well prior to 2k5. Obviously it had a loyal fan base and being on internet forums would tell you that fan base is still there.
Even if half of 2k5 owners weren't happy with their purchase that would still mean 1 out of 10 football fans enjoyed the game. That's a significant number of fans the NFL chose to ignore when they went exclusive.
Notice no other major sports league has done this to their fanbase. The NBA and NHL have currently only one option because of poor sales, not because of a greedy group of owners ala the NFL.