Officially, the stated reasons make sense. At 58, Franklin is probably thinking more about the time he can spend with his family, and I believe most of them live on the East Coast. He's made more money than he'll ever make as a football coach with his increasingly popular Tony Franklin System seminars.
And MTSU sounds like a reasonable place to sail off into the sunset (or wait a year or two before rebooting with a new coach). Franklin had a good year at Middle Tennessee State recovering from his dark few months at Auburn, which eventually led to him joining up with Sonny Dykes in Louisiana Tech.
Unofficially, I wouldn't be surprised if this was a mutual parting of ways.
You might have seen Cal ditch certain elements of the Tony Franklin System as 2015 went along, which I can't imagine pleased Franklin all that much. You might have seen Sonny Dykes being a little frustrated at the inconsistent Cal offense, which would be great for a quarter and disappear for two. And Franklin has been famously tough to work with in multiple spots. It's a minor miracle that Dykes and Franklin have been able to coexist the last six seasons.
There are additional rumors, but nothing has been confirmed and I doubt anything will ever be confirmed. So let's not dwell too much on that.
How big of a loss is it for Cal?
In a recruiting scale, I'm not sure how much it hurts. Franklin is not an active recruiter for most positions besides quarterback. With Max Gilliam already secure on campus, I don't expect Franklin's resignation to have any impact on this year's class.
In terms of everything else? Man. It'll be tough to replace him.
It was a productive few years for Franklin and Cal. The Bears offense was ultimately the highlight of the first three years of the Dykes era, and Franklin did this while juggling multiple responsibilities. Regardless of all the hiccups Cal approached elite offense status in 2016 (13th in FBS in S&P+, 22nd in FEI). As running backs coach, Cal finally showed the best promise they've had in the run game despite losing Daniel Lasco most of the season to injury. The Bears had many great moments this season offensively, particularly in coming back against ASU and beating Texas in Texas.
And you cannot understate the impact Franklin had on Jared Goff. Goff improved from a lanky uncertain freshman to a future NFL Draft lottery pick under the tutelage of Franklin. The two owe each other a lot.
Who will replace Franklin?
I have no idea who it will be. I do know that Cal will probably need a running backs coach and a quarterbacks coach, since Franklin held both responsibilities. Someone on the current staff might have to be let go or someone on the staff will need to take on additional tasks.
One thing I can say for certain: Unless Dykes is comfortable with a total rebuilding year with an entirely new scheme, it will be a coach who runs or is familiar with an Air Raid or spread offense. Expect someone from the Hal Mumme coaching tree or someone has coached under a coach from the Hal Mumme tree. There are options here.
It could also be someone familiar with Dykes from previous coaching spots. The only name that comes to mind off the top is our old friend and current Kansas offensive coordinator Rob Likens. The Cal wide receivers coach was a tough departure to swallow last season, and it felt like the wideouts weren't as consistent as they were the previous season.
And there is a third option: Dykes could have a hand in calling plays himself. Remember Dykes was the offensive coordinator in Arizona during their upswing in the Mike Stoops era, and the Wildcats came pretty close to a potential Pac-10 crown in 2009 (hi Shane Vereen!). A Dykes/Likens partnership might also work, where Likens calls the plays and Dykes has a hand in drawing up the offensive gameplan.
Finally, stop with the Jeff Tedford/Ron Gould talk. It's not happening. Dykes is an Air Raid guy, Tedford and Gould are pro-style, and any return of those two would start the rumor mill going about Dykes's future at Cal.