I would disagree that Nemesis is a decent movie. It is indeed a terrible film for all the reasons you've listed above. Its primary fault is that it is too derivative of previous Trek films and TV shows. It is a mashup of Trek 2, 6, Mirror Mirror, Datalore. It is a movie that tries to service the fans and service new viewers, yet fails in that it contradicts elements of the established series (a Soong prototype nobody knew about,) and glosses over characterization that new audiences would need to have any emotional investment.
But, allowing emotion to enter into the conversation, Nemesis' sin is that it has no heart, no soul whatsoever. Data sacrifices himself and, as an audience member, I do not care. It's just so arbitrary and unnecessary.
If it hadn't been released so close to Two Towers, it probably would've made more money, but not enough to make it successful. The audience was never there for TNG movies to begin with.