The only thing I can see keeping him there is that he lost the NC game a couple of years ago, and he might feel like he needs another crack at it before he leaves tOSU. I don't know who he has behind Fields though - maybe he should just do it this year and then jump.
He has a highly touted 5-star quarterback coming in next summer. CJ Sroud is probably who will take over when Fields leaves, and he should be a really good player for them at QB.
What may hold him back is the stability he has at Ohio State when it comes to family. I know he has young children, so he may want to stay put for a while.
Day has also never taken Ohio State to a national championship. They lost to Clemson in the CFB Semis last season.