People loosing interest in Star Trek is what killed it.
Ratings and interest were already dropping sooner then Insurrection and Enterprise. It's so simple to point a finger at one series, or one movie, and blame that. But it's not as simple.
Insurrection, strangly enough, was far more Star Trek/TNG then First Contact was. Wether or not you agree with it, it had a moral dilemma. Not a very strong one, but still. It tried to resolve issues without boomboomboom and fancy lightshows from phasers before adding boomboomboom and fancy lightshows from phasers. First Contact had 2-dimensional bad guys and far more action then any Trek film to date. The plot itself could have been a generic SF/Action flick, and didn't particulary scream Star Trek.
No, blaming Insurrection is a simple answer to a difficult question. What killed Star Trek, was a franchise that was stuck and unwilling to change its format to a more 21st century way of tv. They wanted to, with a fresh new concept for Enterprise, but ultimatly that was just Voyager Redux. And Voyager was also stuck in repeat mode, doing the same thing over and over again.
The Borg, time travel, and space battles don't seem like Star Trek to you?
And how did INS resolve it's issues without "boomboom" and "light shows?" The second half of INS is mostly an action movie, complete with Picard/Ru'afo fight at the end.
The only Trek movies to not follow that formula are TMP, TVH, and (arguably) TFF. All of the TNG movies were action movies. INS was no more cerebral than FC, and the story was a lot stupider, including the lame dilemma.
Turn the Borg into 'a cybernetic alien race' and the rest could be any SF story.
From the first moments of FC, everything was about action. Only during the second half of INS, did Picard resolve to fighting. He tried diplomacy, he tried talking. He only used violence as a last resort. He talked to Galatin, instead of kicking his ass, reasoning with him. The Enterprise only fired it's phasers twice I believe.
And the dilemma is, in my humble opinion, far from stupid. Why? Because even now, more then a decade later, we are still discussing it, with different opinions flying back and forth. I'd say that's what Star Trek was all about, no??