Day has had 4 excellent seasons, with two NY6 bowl wins, and a national championship game appearance, and he has done a fine job of recruiting top notch classes.
He also did a good job of keeping the team together after Urban Meyer got in trouble (again).
He has at least a couple of mulligans he can use, assuming he has a substandard season. I see no reason why he can't get to a 10+ win season, though.
To put it this way, he may have started out inheriting the keys to a Ferrari that Urban Meyer left for him, but the fact that he has put in four quality seasons in a row shows that he's not some kid with a silver spoon in his mouth. This team is mostly his players, and they've been contributing heavily towards the team's success.
I'm not about to put him on the same level as Nick Saban, but if he keeps this up, he could very well get in that territory sooner rather than later.
The only reason why I think he would leave, is if a NFL team came calling and offered him a heck of a nice package, and if that were the case, I think he would make a far better pro coach than Meyer did when he stunk up the field in Jacksonville.