Firing him now has no downside whatsoever. Keeping him on has one glaring downside: there is a very small, but non-zero, chance that he wins two games and makes it to a bowl game and make himself hard to fire. Firing him has one glaring upside: there is a small, but non-zero, chance that the interim coach wins two games and makes it to a bowl game and makes the program/job look more appealing to others.
Plenty of people will roll their eyes and say, "Yeah, whatever, 5% chance, who cares?"
You don't succeed in the long run by giving negative freerolls.
On a personal level, I know it's in the best interest of the program for him to be fired, so I have to root against the team until he is fired. If he's fired now, I can root for the team to hit that longshot and make a bowl game.