RB longevity is not just a function of total touches, whether you count college or not. You need to look at high water marks. One high touch season often leads to decline. Whether that is just physical or maybe mental would vary from one player to another. Big contracts often follow those seasons. Funny things can happen when a guy reaches his lifelong goal.
Here are the historical league leaders in touches.
https://www.pro-football-reference.com/leaders/touches_year_by_year.htm
Looking at the last 10:
2019 - McCaffrey in his 3rd. season- All Pro - injured in 2020
2018 - Elliott in his 3rd. season - Pro Bowl - yards per touch dropping from 5.3 to 5.0 to 4.8 this year
2017 - Bell in his 5th. season - All Pro - no need to recap that
2016 - David Johnson in his second season - All Pro - missed 15 games in 2016 with decline to follow
2015 - Peterson in his 9th. season - freak of nature recovery from ACL - All Pro - missed 13 games the following season with decline to follow
2014 - Murray in his 4th. season - All Pro - good season in 2016 between off years in 2015 and 2017, then retired
2013 - McCoy in his 5th. season - All Pro - he kinda broke the mold - he had several good seaons after that though nothing to match that All Pro year
2012 - Foster in his 4th. season - All Pro - missed 8 games the following year, followed by a good season, then finished. Second time was not a charm; see 2010 below
2011 - Jones-Drew in his 6th. season - All Pro - missed 11 games the following year then sharp decline
2010 - Arian Foster the first time in his 2nd. season - All Pro mold breaker given what followed
In short, if you paid these guys top money after those seasons it would have been a mistake with the exceptions of McCoy and Foster the first time around. McCaffrey is following that pattern.
These numbers don't include playoffs and I'm not going back to figure that out. Some of those guys were not so burdened. There is a clear message. If you're contemplating an All Pro or Pro Bowl-caliber running back as a potential signing, here or in your fantasy league, add up the touches including playoffs and if they are pushing the high 300's or into the 400's, you ought to think twice about the cost vs. decline risk, injury or otherwise.
Derrick Henry is the current cautionary tale, on pace for 428 touches. I believe Josh Jacobs is second on the list, on pace for 387 touches. Mixon is right behind. With these guys it's just a fantasy issue since none are in contract years.
Aaron Jones is on pace for 320 touches. Not bad and in light of his college + pro totals especially not bad. Pile 3 or 4 playoff games on top of that on the way to a Super Bowl appearance and there will be a serious question to be asked on top of the already serious questions.