That's the point. Zeke is the engine, but do you commit that much long term money to 1 player in a fairly high risk position?
Depends on what else you can do with that $15M.
Just off my head - what does $15M for offensive line get you? Nate Solder and Trent Brown just signed for about that much as tackles, and neither one's that good. The WR market is inflated as heck; $16M gets you Sammy Watkins or Allen Robinson, and neither one cracked 1,000 yards last year. $9M only gets you freaking Cole Beasley. For edge rushers, $15M gets you third tier guys like Preston Smith or Za'Darius Smith. $14M for safeties will get you Landon Collins or Earl Thomas on the downslope of his career.
On the positive side, I'd love to keep Jaylon here for $12-ish million. Top dollar for a cornerback is about $15M, and that's about what Byron would ask. Byron vs Zeke would be an interesting discussion. As far as recent DT contracts, I'd love to have Sheldon Richardson here for $12M.
But all in all, $15M for a top shelf running back isn't crazy. I think Zeke has a greater impact on a game than anyone on that first list, excluding a healthy Earl Thomas.