As someone else said in the comments, maybe have the 7th movie set in the present, during the war instead of time travelling. The Terminator franchise couldn't last much longer, especially if it continues with the time travel plot. If Schwarzenegger appears for one final movie it could wrap up to the story, even if it just focuses on the evolution of each terminator model, Schwarzenegger could still make a brief appearance.

I haven't seen Genesis or Salvation yet, so I can't comment on them or their storyline, so I don't know if this has been covered in those movies.