Over-saturation of Trek might have been part of the problem. But another part of the problem was that they had stopped taking chances with the property.
For the most part, episodes of Voyager and Enteprise could have been episodes of TNG with a little re-writing. both of these seires had initial premises that could have lead to new and interesting story-telling, and they seem to have mostly wasted these opportunities. I will, however, give Enterprise credit for trying real hard in it's last season to turn this trend around.
Agreed. It was more lack of quality versus quantity. I'm a huge TNG fan but Voyager was really just seasons 8 - 14 of TNG (story wise). At least DS9 tried something a little different. TNG was a reflection of the late 80's and 90's but the series after that really didn't try to change that much. While ENT started to get better the last season, it was too little too late. It didn't help that the movies weren't too terribly entertaining either. They were more like a two hour special episode.