Then why did the 2009 film make nearly as much money as all four Next Gen films combined?
1) Marketing. JJ-Trek was advertised everywhere whereas Nemesis came and went with only us noticing.
2) Over-saturation of Star Trek. Dunno about USA but here in Britain TNG, DS9, VOY and ENT were constantly on television. BBC2 was flooded with Trek and Sky One may as well have been renamed "The Star Trek Channel". The audience needed to miss Star Trek until they'd pay for it.
You don't pay to go see a hooker if you've got your sexy wife at home that'll screw ya for free. You only do it if you're single and there's nothing else available.
Your reasoning simply does not work... First Contact
premiered while both DS9 and Voyager were on the air and TNG was in reruns here in the States and it had no issue selling tickets. It was a success because it took something that was popular in the mainstream, zombies, and put a Trek coat of paint on it.
It's like saying The Final Frontier
failed because Next Gen was on the air, which is what Shatner said. The quote was something along the lines of "who looks forward to turkey when they're having turkey sandwiches everyday", which was horseshit then and is horseshit now.
Modern Trek simply couldn't sustain itself in the long run. It wasn't that it was terrible but it never changed.