I'd say oversaturation of star trek did in nemesis.
First Contact was released when we had Deep Space Nine AND Voyager running at the same time, and it came out two years after Generations. Did audiences or fans complain that there was too much Star Trek? No. First Contact went on to be the most successful of all the TNG films both critically and financially. Nemesis on the other hand was released four years after Insurrection (At the time the longest gap between movies) when there was only one Star Trek show (ENTERPRISE) and was a failure to both audiences and fans.
In conclusion, not only did over saturation NOT kill Nemesis, there was no over saturation period. The film was just bad, and John Logan played a very important part in that regard.