I don't think the TOS movies were better than the TNG movies, on the contrary. Every movie in the group VI - X was better than anything in I to V, and the worst of the pack is TWOK. Nemesis had at least a fine space battle, while the FX in TWOK are the stuff of nightmares.
In hindsight, the standard for SF on the big screen was maybe set by Alien in 1979. TMP was a greater box office hit in the same year, and since that was a lulling kind of movie, it's success can only be explained by the faithful fans of Star Trek. Paramount obviously believed the fans will take anything, and produced the string of poor movies which II - V are for the better part. I actually liked every one of them when they were new, but everything has moved on for 30 years.