Seeing as Gattis was only credited with recruiting one player this cycle, it didn't do much good to keep him if the plan was to replace him before next season. Even if the next OC is still coaching with a title contender and didn't want a distraction, it probably would have been better to just fire Gattis weeks ago to show recruits (especially at WR and QB) that they can expect a different offense next year.
If I'm a recruit, I'm not going to put any faith in a HC telling me that there will be a different OC. We just saw Mario go through this last year when his top choices (supposedly) turned him down and he settled on Gattis. If/when it happens again, what's to stop Mario from deciding that he'd rather roll with Gattis again than go with his 9th or 10th option? Or maybe he just remembers that his original plan was to have Ponce eventually replace Gattis in a couple years anyways, so he might as well make the switch to Ponce now.