When a series that has a large gathering of fans over many decades does an about face on theme music and ambiance, then of course there's going to be friction. It has nothing to do with the song itself, and everything to do with change.
Further, it introduces the show as a drama. Now, we all know it was a drama in the first place, but at least that was all hidden behind cool space sounds, pseudo-technical jargon, hot aliens, phasers, and so forth.
To explicitly state, "Hey, you know, this is a tale of friendship, alliances, betrayal and hope" right at the beginning with a silly 90210 theme song? It's just not going to sit well with the fans.
As an aside, why does everyone generalize?
---------- Post added at 04:20 PM ---------- Previous post was at 04:13 PM ----------
Also, it's wistful. Looking back and yearning for times gone by.
How much more powerful could it have been if the entire theme was a hopeful desire to move forward and seek out the future? To boldy go where no theme has gone before?
But I've never seen the show, and I'm quite certain if I had I would have grown used to it.