We shouldn't be blinded to his history by the elite level run that he went on to close out the season. History is littered with backs who looked elite for a short time, in a contract year, who then reverted to the mean once they got paid.
Top tier running backs don't need to split carries to make it through a season. Top tier running backs top 4.8 YPC. Top tier running backs don't need four years of coaching to figure out you can't dance backward behind the line. Top tier running backs are breaking 1,500 rushing and 2,000 total yards per season. (scoring can't be used here due to the way they let Charbs finish drives)
Walker doesn't belong in that conversation, and if you are paying to have two backs split carries, neither of them should be paid top tier money.
If Walker had a couple of seasons approaching 1,500/ 2,000 and 5 ypc, where he made it through the entire season as the bellcow back, then it makes sense to pay him in the 15 mill APY range.
He may be him, but so far he has only flashed potential, not realized it for a full season.