The Alien, Terminator, and Matrix franchises have similar problems. Aliens successfully changed genres, from horror to action. But subsequent movies could never recapture the primal horror of the original or the fun action of the second. It's almost like there are only two local optima in the Alien movie universe and Alien + Aliens took them both. Terminator is the same. The first movie was a perfect sci-fi action mo…
Wrath of Khan - Star Trek does a Shakespearian tragedy.
The Voyage Home - Star Trek does a family-friendly time-travel romp.
The Undiscovered Country - Star Trek does political allegory.
And just like The Matrix, later films do not exist.