You could make the same argument that the first Terminator didn't need adding to or expanding, but I'm still glad they made Terminator 2. The trouble is, at its core the franchise is such a simple and straightforward story that I think you need to do more with it if you want to keep making more movies (which arguably the second movie did pretty well with its challenging of the closed-loop ethos of the first movie).
I've enjoyed the various non-Cameron spinoffs and sequels to varying degrees too. The Sarah Connor Chronicles TV show was actually pretty good once it hit its stride, and explored some interesting alternate story ideas that grew pretty naturally out of the basic building blocks of the franchise. I wish it had carried on for longer.
Terminator 3 had some terrible moments but also some pretty good ones, and I liked the ballsiness of its final twist and ending.
Terminator Salvation was... a film that was released. I can't think of anything better to say about it than that.
At this point, I think the best option for the franchise - if it really has to continue - is probably a completely fresh reboot. Strip away all the clutter and history of the series - even Arnie - and get back to what made the first (and second) movie work so well. The human story of the saviour of mankind on the run from an unstoppable killing machine.
It risks being repetitive/derivative of the original and basically unnecessary, but I think it's better than the alternatives.