It's perfectly been pointed out that DS9 did not have the audience.
The true crime is that we never got a crossover movie with TNG/DS9/Voyager cast members. (and I loathe 80% of Voyager)
I would hope it would've been a better plot and script than Generations, but from a fanbase standpoint, a movie with all or even combined partial casts would've been such an
EPIC
STAR
TREK
EVENT
I tend to disagree. I like Generations script, first of all, despite the writers apologies. Showing Picard without his life in space--through symbolism of losing stars, losing Enterprise, and showing Picard failing before entering the dream machine that is the Nexus--sets up the rest of the movies. Picard's damage is there, in Generations, which informs the First Contact emotional elements, gives Data emotions which helps move him forward and mirror certain aspects of him, let's him heal with the Ba'ku the way he healed with Robert, and informs what he wants, context for the darkness within, and sharpens Shinzon as the Nemesis that brings down Picard.
These stories are less about space battles and more about charter, theme, symbolism, humanity, psychology.
When they went epic, the show was stupid. In its earliest stages they need a simple connection. Like building a story about the loss of innocence of their first dreams about space and exploration. Sisko is taken from his home, twice, by the stars (Jennifer and the home with Kasidy). Voyager is lost, Kathy loses her dog and husband, and the time spent in the Delta Quadrant. Picard becomes a mass murderer, loses control of his mind, his potential family.
So, what is the arc from there? Not something as stupid as a support group, but an enemy that leaves one bitter, one healed, one searching for which reaction to have.
That type of story is a head-scratcher. So, they'd probably have a Borg showdown with equal times of them playing hero.