The problem is, you're going by contract averages.
Next year his salary is only $8M. He may not be worth that either but $5M of that is guaranteed meaning the Rams are paying that whether they cut him or keep him.
So the real question: is he worth $3M for his contribution to the offense? I think it's too soon to call but there's definitely evidence that conflicts with the arguments you list against him.
I don't think it's accurate to say he's relegated to scat back duties. In week 7 he had 19 snaps which is the most to date this year. Several of those he lined up and was targeted as a WR.
Other than week 6, his usage has been steadily, albeit slowly, increasing.