The studio’s desire to make Star Trek VI the last original series movie was, in my opinion a terrible marketing move. Star Trek VI would be the 15th highest grossing film of 1991 and have the best opening weekend of any movie up to that date. Star Trek VI had a worldwide take of around a hundred million dollars from a twenty five million dollar budget. This profit excludes VHS, DVD and now Blu-Ray sales.
Meanwhile TNG was acheiving massive ratings on TV and interviews from the time suggest that none of the cast wanted it to end. Furthermore, when they did make Generations, it was done on the cheap.