Ryan Day can develop QB’s and he clearly knows how to run an offense. The offense will likely be a top-10 unit as long as Day is our HC, and I'm not concerned about that side of the ball no matter what coaches depart as time goes on.
The challenge for Day will be replacing the defensive coaches that leave for other opportunities, or retire, and I think that his ability to pick the right replacements will determine how successful he is long-term more than anything else.
Every head coach has more of a background on one side of the ball, and they'll often give a coordinator (almost) total control of the other side. This allows them to primarily focus on what they know best, even while being a head coach who needs to oversee everything to some extent.
The problem is that, unless you're lucky enough to find an elite coordinator who is perfectly content not being a HC (like Swinney has with Venables), you'll probably have to bring in a new coordinator for that side of the ball every couple years or so. This isn't exactly easy, but one coach who's done a tremendous job with his replacement hires is Nick Saban.
As we know, Saban is more of a defensive guy. He's been at Bama for 12 seasons now, going on his 13th, and he's had to replace good OCs pretty frequently. It seems like he always does a great job with it though. Here is everyone who has been Bama's OC since Saban has been there:
-2007: Major Applewhite
-2008-2011: Jim McElwain
-2012-2013: Doug Nussmeier
-2014-2016: Lane Kiffin
-2017: Brian Daboll
-2018: Mick Locksley
-2019: Steve Sarkisian
Only questionable OC hire he made, to me, was Applewhite in his first year at Bama, and I only say that because Applewhite was so young at the time.
Some were better than others, but they were all perfectly qualified guys to run an offense. He's also had a lot of great position coaches on offense since he got to Bama like Mario Cristobal, Billy Napier, and Burton Burns.
Bottom Line: Saban’s ability to consistently put together quality offensive staffs that he could trust is a big reason Bama has been the most successful team in college football over the past decade. It's allowed Saban to spend as much time working with his defenses as he wants to, while also being able to effectively manage games and prepare his teams the way a good head coach always should.
It’s not possible to upgrade 100% of the time when replacing a coordinator or a position coach, but it’s definitely possible to avoid a significant downgrade 100% of the time at a school like Ohio State. We've seen how detrimental poor replacement hires can be to a team.
I think if Day can consistently make the right hires on the defensive side of the ball when a replacement is needed, and avoid letting personal relationships affect these decisions more than they should, we could be in for a very long streak of CFP appearances and get back on par with Bama and Clemson sooner rather than later.