It is the destiny of all franchises to turn into action comedies: Terminator, Batman, Alien, Friday the 13th, Nightmare on Elm Street, Star Wars, James Bond, you name it.
With that said, Terminator 2 was amazing, but for different reasons than the first, because it was a completely different genre. Switching genres usually kills a franchise (and leads to relaunches like Batman, Bond, Friday the 13th), but it kinda worked for Terminator 2, because the whole first movie is about the future, and hinting that in the future there is a GIANT ROBOT WAR, which is action city, so it's kind of hinting that it will turn into action.
I loved the first 2 and hated the 3rd. I also thought the 3rd movie ruined the chances of there even being a 4th, since it explained too much about the future, including John Connor's death. If I know that he dies, why do I care about his next adventure? I'm only watching this for Christian Bale and giant robots. I haven't seen a good non-animated giant robot in a while. What was the last good giant robot movie? I thought the 3rd Matrix sucked.