I'm just going to cut to the chase and say that what went wrong was that...time simply passed.
It's actually something I'm having to learn at my current stage of life. (I'm 35--not old yet, but not all that young anymore, entering the mid-section of life.) This is not directed at anyone reading, just some cosmic philosophy to chew on. You think everyone's at the same pace as you, everyone's keeping up with what you are, everyone around you has the same perception of the passage of time--then one day you realize it's just you.
I'm one of those people who, perhaps incorrectly, still doesn't feel like any time has passed since high school, when for me TNG was mainstream and everyone watched it like any other show on TV. I thought my friends and family graduated to DS9 and VGR. They didn't. My friends and family don't know that INSURRECTION and NEMESIS exist, much less that there was a prequel series called ENTERPRISE. Additionally, most people don't have an attention span that can last 15 years much less 30. People move on. It's the human way. The nature of the beast. People want to experience a variety of things during their lives and may not adhere to a single adventure.
Some of us, such as myself--geeks like me--we have no sense of "ago." It's still 1979, 1989, or 1999 in my head. I can be anywhere in time I want to be. I remember being so excited to see RETURN OF THE JEDI at age 9 in 1983 and wishing, then, that I'd been old enough to have seen SW in theatres in 1977! I remember the BACK TO THE FUTURE's and the INDIANA JONES's and being so excited for each of those. I remember the vibes surrounding each section of the STAR TREK franchise at the time, series or movie. People forget how huge STIV was in 1986, for example--it was a runaway blockbuster hit that was the firing pin that allowed Paramount to greenlight TNG in the first place. People forget the rocky year leading up to that. To me, there's no concept of time passing. But I must realize that there is for most humans--people want to "progress." They can't keep doing the same things. I'm content to continue with DOCTOR WHO, for example, something that started in 1963 in England. It's still going. I'm still keeping up. It's a lot harder for most people who, I must face it, have *lives*, have *children*, have *careers* and who have needed to move on--to other places, to other spouses, even to other families, etc.
I'm a geek. I live for TV and movies. They set me free. Real life is too boring. Movies are real life with the boring parts removed. I'm an escapist. (An escape artist?) I'll close with one example of two separate lives. In the relatively short time since FIRST CONTACT (1996), I have eagerly awaited each installment in the TREK canon, series or movie. I lived the SW prequels, one by one, all the hoopla and hype. The LOTR and HP films, one by one, all the hoopla and hype. I did the midnight release parties for the merchandise, etc. I have loved LOST from Day 1 and look forward to it each week. That's how I define the last 13 years. In that same space of time, my *younger* sister has achieved a Masters in Microbiology, gotten married, moved around and lived in three states and had two children. I'm not a failure in comparison, nor do I consider my life less "real." It's just the difference between people. I've always lived in my head, because fantasy is more fun than reality, and that's a big place to live in.
Maybe I haven't answered the question correctly: Where did it go wrong? I think I've tried to demonstrate how hard it is for any long-running franchise to hold onto its fan base. We think the new STAR TREK film has it, the franchise is safe in Abrams' hands, etc. But in no time, in only a few years, this vibe will disappear. 2009 will give way to 2015 and 2019. This cast will age. These fans will tire and wander. Life goes on. It's just that some of us are aboard for the long haul and others aren't. It's all about perception, and the only culprit is time itself.