TNG, DSN and VOY weren't really flops. Sure by the time ENT came around the audiance had fallen to a level unable to support this but :-
In the case of ENT bear in mind how many seasons of Trek had been on. Many consider TNG and DSN to be the best of the modern era Trek shows. That isn't to say VOY and ENT are bad shows, but for example if a portion of your fan base is referring to VOY as TNG-lite you have a problem. Shows are compared to their predesscor and if the fans think it isn't as good they might leave. Or to put it more simply it wasn't quality TV anymore.
TV audiances can diminish over time