It doesn't have anything to do with getting "better" -- and everything to do with "bettering" his draft position.
If he's a 6th round pick, 7th round pick, or an UFA, then there is a chance that he might not even make a team or earn a professional contract. If he returns and has another big year, then he'll elevate his stock and go much higher in the draft. The difference in $$$ would be immense.
Personally I think he's going to be an NFL starter, but I'm pretty sure if you asked experts who haven't seen him as much and aren't at all biased, the most likely outcome in their opinion would be late round pickup, maybe out of the league after the rookie deal, maybe a career backup.
A late round rookie deal is worth a little over $1M/he for four years, non-guaranteed. The minimum salary after that is $800K, top backups make more.
So there's definitely a case to be made that a fifth year here making $1.5M or so in NIL is likely to be his highest paying year of football left, and if he ends up being a one contract guy, that $1.5M could constitute more than a quarter of his potential remaining football earnings.
Again, personally I think he's a bargain as a late round pick and likely to be a starter or at least high end backup (they make a couple million a year), but that's a gamble to an extent.
I'm not sure how high that improvement ceiling is for McCord. Like I said, the guy just led FBS in passing.
Doing it against better defenses could go a long way.
Snap counts mean absolutely nothing to the NCAA.
The best argument for the NCAA is something like, "The plan/expectation when he went to Ohio State was that he'd redshirt, and Day messed up. Protect McCord's earning potential and send a message to other kids that if a coach makes a silly mistake with their redshirt and they didn't gain material experience as a result, they will be protected too."
Still not super likely to work, but I think that's the best argument from the NCAA's perspective.