I'm just going to flat out say this:
Enterprise will never reach the climax of popularity then its predecessors have.
Don't get me wrong. I enjoy Enterprise, and I will watch next Wednesday's "The Xindi", and see if I'm going to keep watching. I feel that B&B need to be ousted, and someone with a little more guts need to move in. However, I also think a lot of the reason the show does not take chances is the executives in charge. They feel they need to keep it overly safe, and it hurts the show. I hope that all of this will change with the upcoming third season.
Anyway, back to my main point: Enterprise will never reach the climax of popularity then its predecessors have.
Why?
I really don't think I can explain it for Classic Star Trek, since I was born in 1979 and as John Sullivan says, I don't know any better, because I didn't live it. Well, honestly, as much as John and I are buds (we are, right?), I think his opinion in this regard is wrong.
Yes, I did miss the height of Trek's popularity in the '70s, but I was raised on The Next Generation, and watched week after week. I remember that our FOX affilliate aired reruns of TNG every night at 9pm CST, and the original series at 10pm. On Tuesdays, we had a full night of Star Trek, hosted by a guy in a Classic Starfleet uniform who called himself "Rusty Hammer", and was the janitor aboard the Enterprise. It was goofy, yes, but it was the height of Star Trek fandom for me. The 25th anniversary was huge as we had specials, movie marathons, a Best of Trek marathon. It was just fantastic. I couldn't go anywhere without seeing Star Trek that year, and I loved every minute of it. Of course, I was teased a lot in school, but oh well.
Then, DS9 came on the air. At the time, it was good, but wasn't great. I've really begun to appreciate the series as I've gotten older. Whereas TNG was more of the sterile 24th century, this show had shades of gray. It developed characters because it had to. They were on a space station and couldn't go anywhere, so instead of exploring the galaxy, they explored themselves. That's what made DS9 great, in my opinion. I think a lot of fans are picking up on it now that its on DVD, and its certainly now finding a niche and becoming a bit of a cult classic. It wasn't as popular as TNG, and it never will be. Voyager was the same way, and it comes down to one simple point why the popularity was and has been on a steady decline:
Overexposure.
Paramount killed the rise of Trek by overlapping TNG, DS9 and Voyager. Not only that, but Voyager, in my opinion, was just not a good show. It had a fantastic premise that was ruined by laziness and technobabble. It could have been glorious.
Enterprise is feeling the leftover effect of that situation. It has the prestige of being the only Trek show on the air, and while it could certainly use a boost of good writing, it's better than Voyager. But, the audience is pooped on Trek right now, as are the producers, which is why while DS9 WAS fresh, the last season of TNG and all of Voyager suffered from problems, and while Enterprise, while I believe its better, still suffers from some of the same problems.
Enterprise could and in my opinion still can work, and it could have been very popular... had it premiered today instead of coming right off the curtails of Voyager.