Zerith already is canon, the only people who ever say otherwise are the people who don’t think Crisis Core is canon.
But Crisis Core is canon, and the moment they say stuff like “ignore Crisis Core” (which I’ve actually heard them say), they’re basically admitting Zerith is canon.
SE didn’t care then, they don’t care now.
For the more casual player the question is not whether Zack and Aerith were a thing, or even whether Aerith has a thing for Zack at the start of FFVII, they were and she does, only the most rabid shipper would deny it. The question is whether Aerith gets over Zack and falls for Cloud instead. When that question pops op they’ll inevitably quote maiden, which basically shows you how delusional you have to be to still hold onto the idea that Aerith prefers Cloud over Zack, when your ship requires you to pretend that Crisis Core isn’t canon, but Maiden is.
Personally, I don’t think it matters much, it’s not impressive to beat a 5 year old memory. Even if Aerith does “get over Zack” through Cloud, that doesn’t mean that she’d have preferred Cloud over Zack if both had been available. Even directly believing you straight up prefer the person you’re with over someone you haven’t seen in 5 years doesn’t mean much, since you’re unlikely to be impartial in the matter.
What matters in the end is that Aerith and Zack are dead, and Cloud and Tifa are not. It took Aerith 5 years to get over Zack, I can assure you that when she’s reunited with Zack, getting over Cloud would take a fraction of that.
SE has two perfectly viable couples here, both with massive followings, both with histories, and both located together both physically, and emotionally.
Cloti and Zerith tie up all loose ends, allowing the story, and the characters to end without anything feeling incomplete. If Remake is an attempt at “finalizing” FFVII, then SE would be insane not to make use of that. Conversely, Clerith leaves both Tifa and Zack hanging from a romantic narratological perspective. In the original game, Zack wasn’t that important of a character, but he is now, people love Zack, and it shows in the remake, they know people love Zack, because Zacks introduction shot has the cinematography and sound design of a returning hero. And what is Zacks current raison d'etre?
It’s getting back to Aerith. For the games to end that quest by effectively saying “that didn’t lead anywhere, Aerith is into Cloud now, and Zacks story ends with him basically just being ok with that” would feel extremely unsatisfying, it would beg the question “why did we invest all that time then”?
Reunion is a returning theme in FFVII, Tifa reunites with Cloud in the lifestream sequence, but we still haven’t seen Aerith and Zack reunite. We’ve seen them afterwards, like in ACC, but the moment where they meet again has sadly been denied to us with the exception of the underwhelming mess that was maiden who traveled the planet. But Maiden isn’t canon and the writing shows it when it has Zack and Aeriths reunion be focused on Cloud more than it focuses on they themselves. Having the lives of all orbiting characters revolve around the ‘center of the universe’ MC is a staple of bad fan-fiction writing.
There is no doubt in my mind that we will see that reunion this time around and done properly. There is no way to do that in a way that’s in character that does not make it painfully obvious they still love each other. Anything else would not just be out of character, but disappointing writing, if you set-up two characters who have been searching or waiting for each other for 5 years, and then not give that a proper pay-off, you’ve failed as a writer.
Zerith was always canon, and remake will show that it remains canon even after death.