I agree about the social universe being crowded, back when TNG was just coming on the scene there wasn't a great deal of competition, but a decade later there dozens of sci-fi shows and of course hundreds of other shows and channels for those shows in general that viewership is naturally going to go down. Add into that recycled stories and themes for later series and almost 2 decades of continuous Star Trek and you get a fatigued series and a bored bunch of people who are no longer that interested when there's so much else to whet their appetites.
In terms the aliens, I do think that for better or worse Enterprise did try and reinvent several alien species and do something different. Was it radically different enough and consistently good enough to survive... apparently not but the effort was there.
As for Trek09 we've seen some big changes I do foresee a successful movie enterprise, but after a trilogy say, ending around 2015-2016 I should think that will be the end for Trek for the time being, at least in terms of the big and small screens.