Sometimes I feel like some people didn't get the point of T2. "There is no fate but what we make ourselves." If time couldn't be altered, then why would Skynet send a machine back to alter it in the first place?! Except for T1, all of the movies have shown that time is being changed.
What some people seem to be forgetting is the groundwork that T2 layed. A machine learned the value of human life and became a protector. A machine taking up the mantle of being the leader would have been the next logical step. If you feel no jepordy for John Connor, THAT is rendering the movie's philosophy pointless. In the beginning of Salvation they gave you the sense that even John thought he might not be able to die, like when he dove out of the chopper and swam to the sub. The movie was on a path to shock you by the end. Hell, I was shocked when John stabbed, but it didn't follow through! 
Secondly, it wouldn't render the movies pointless because more importantly than protecting John, was protecting the idea of John. Someone who could be  a hero or leader and rally the resistance to eventual victory. 
And how bad ass would it have been to now have a John Connor that is a cyborg, (he still had a human brain and organs) in the sequels. That would have made for some intense action.