. Just like old Spock can't get back to the old Star Trek time line, because, at least from his perspective, it's not there anymore.
Realistically, the problem is just bad writing. Hell, even using the terminator examples, they keep using the setup that one side sends someone, and then the other side finds out and sends a counter. Can't really work that way, as the first guy either succeeds or fails instantly from the future perspective. If Skynet sends a terminator, they either kill Sarah or not, and John can't send help. It either worked, and he's dead, or it didn't, and no need to send help. Once you've started from that piss-poor starting position, why bother to continue with the logic? Based on the first movie alone, seems like a closed loop, but not sure what the *different* situation would have been to create the first itteration of that loop. And again, even in the closed loop itself, the time travel logic doesn't work, because only the first time traveler has the chance to complete their mission. Unless Skynet and Connor agreed to use the time travel device together, just for fun...?
As for the Spock example, it's still quite fixable. And he's smart enough to know that, aside from the writer's desire for him to NOT fix it. Let's look at it this way:
Yes, he can't travel to the future and correct things (hell, Back to the Future 2 explained this just fine). What he CAN do, though, is go backwards from his current timeframe and correct things there so that his future DOES exist again. Pretty easy, actually. He can spend all the time he wants creating a new super-weapon (or more squishy red stuff), and then just travel back in time to the point that Nero emerged from the rift. Spock knows exactly when and where to be, and can be waiting for him. he then just nukes his ass, and warps back out before George Kirk and company arrive. All fixed. Spock then is free to travel back to the future he remembers, as the only change would be that George Kirk notes a weird space storm in the log, that quickly disapated.
Hell, and that's with 2 seconds of thought, and not using any weapons or tactics that Spock hasn't already used previously. And 100% doable with the technology at hand. Then again, the problem was never the ABILITY to do this, but that the writers wanted a kewl new timeline to play in, so they could rehash old villians and situations rather than make up new names...