IIRC, the robots found another time travel facility at the start of the second movie.

You make a good point about Reese saying he's from a "possible" future, except that it's a major kick in the movie when you get to one of the last scenes and see that, in this movie at least, the timeline is, in fact, immutable. :-) You may be able to go back, but you can't change it; and at worst, you'll make it happen. Which is what Reese did. I liked that.

Wade.