I don't think he has slowed down at all. I get it many here would dump him on the trash heap tomorrow if you could. Fact is he has not dealt with injury and the whole BS of all RB slow down if they carry x number of times is stupid. Those they point at all were heavly impacted by injury not their carries. If choose to believe otherwise well that is fine I don't buy it. Zeke total yards last season was his highest total and 4.7 a carry is still an impressive number at RB.
Look it up, there's data for it. Just a few names from the last decade that came to me off my head:
Shaun Alexander blew out his knee after about 1700 carries, and was out of the league after 2100 carries.
Edgerrin James was released after about 2200 carries with the Colts, and he never went over 4 YPC again.
LaDanian Tomlinson had about 2300 carries before he started rushing for under 4 YPC.
Steven Jackson made it to 2300 carries before he left St. Louis, and was completely washed when he went to Atlanta.
Burner Turner made it to 1700 career carries and was totally done by the end of it, although in fairness the Falcons gave him multiple 350+ carry seasons that are known to drastically shorten a running back's "lifespan".
Maurice Jones-Drew broke down around 1700 and was out of the league with 1800.
San Francisco let Frank Gore walk after 2400 carries, although he's still going with an astonishing 3,384 career carries.
So on the whole, 2100 carries plus or minus a season is a good historical barometer for an RB's peak shelf life. None of those guys were injury prone; I left out guys like Brian Westbrook and Arian Foster who were pretty much just injuries waiting to happen. Aside from Alexander's knee, they all kind of just wore out. Except for Gore; Gore's got that Favre / Emmitt gene where he just ain't fully human.
There's nothing magical about the 2100 figure, it's mostly just tied to age and workload, but for a 300-350 carry a year kind of feature back I think it makes a decent estimate.