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.