I think that theory holds some weight especially with Trek's strongest periods coming in the front-end syndication eras (TNG, DS9).
UPN is definitely the primary reason why Enterprise was cancelled. The network is a cinder block. I think Bakula's comment was very true when he said the irony was that Enterprise will still be watched on DVD (or whatever we have) in 20 years while most of UPN's other shows will be forgotten.
I'm not going to say that Enterprise was always brilliant because it had bad episodes but every show in the history of television has done bad episodes. But I will never believe that Enterprise's quality was the over-riding factor in its cancellation.
I don't care what anyone says. I guarantee if you put the greatest episode of Trek in Enterprise's time slot with next to no promotion and on a sixth rate network, it would do no better.