Star Trek IV: The Voyage Home is said to have had the most crossover appeal and was one of the most financially successful films in the franchise. So why couldn't they bow out with an "exploration" themed film? Even ST:TMP was a hit, though that may have been based on the ten-year wait for a Star Trek movie. But it could work or even be balanced with a antagonist in the story. I could be argued that Star Trek VI - TUC kind of met that balance with its presentation of a seminal moment in galactic history confronted by a gang of reactionaries drawn from all sides.