Not canon to me in all likelihood, but possibly fun.
Canon, shmanon. Nothing is canon except whatever movie you're currently watching. I sure don't watch T1 and think that Joe Morton is right off camera waiting to pick up the Endo arm or that Reese was "wrong" when he said "it's just him...and me." I just watch it as this awesome 1984 sci-fi/horror film. The end. When I watch T2 I see it as a 1991 take on the story, with T1 as backstory but is otherwise a fun event from 1991 that still holds up today. Genisys will always be old Arnold returning to form (as much as he can at 68 anyway) with a 2015 vibe to the whole thing and the closest I'll ever get to seeing that future war prologue that never made it into T2.
I hardly ever marathon movies and their sequels in one sitting as some big giant "canon story." I might try and watch two SW or LOTR films back to back but that's pretty much it,
ever. It's not like if you enjoy Genisys, and put the blu-ray on your shelf, that some uniformed Canon Police are going to show up and take a mug shot of you and report you for liking the "wrong" combination of films.
Even T3, which suddenly sucks a lot more than I thought it did a few days ago, I'm sure I'll revisit if I'm ever in the mood for "early 2000's" Arnold. It sure is better than The 6th Day and Collateral Damage and whatever else he did at that time.
By the way I'm actually saying this as someone who really did think that "canon" was important and now I just don't anymore. If films were shot back to back by the same director then I treat them as "one story." Otherwise they're all just different "takes" on a certain theme or narrative, even if one positions itself as being a continuation of the last.
The closer a sequel comes to not contradicting any of its predecessors the more impressed I am with it (but only on that one level) but such contradictions are never a deal breaker if I truly enjoy the film on its own terms.