Late to comment, but I still don't really understand the notion that Arnold being back again automatically = bad, especially if we still have Cameron again among other things.
Hard to say when Arnold will actually retire but he's only got like how many years left at his age? If anything let him take his Terminator role with him to the grave before the series moves on without him... Then again, we might get CG of young and/or old Arnold after he's gone which I actually would be against.
Ah. Yeah, I can agree to that at least. He had a proper sendoff in T2 but since the series later still continued because moneyz, give him one more final sendoff in the form of a Logan esque ending film or something to the T-800 (well, a T-1000 now if you count Genysis)...
I don't have a problem with him being back again in hindsight if he's under a better script or Cameron or something, but can agree that he needs to finally go at some point.
Arnold was the only good thing about Genisys, though. Personally, I'd like to see him as an old human soldier in the future war, whose likeness the 101 model is based on. That way you could have Arnold being badass without going through the "T-800 learns what it means to be human" motions again.