The team could easily have managed his $4M cap hit this year, and next year it actually drops to $2.7M. Factor in the idea that he's not getting the lion's share of the carries, so even if he's your starter in 2018, then you have him cheap.
I'm sorry, but I don't think it makes sense to assume you can replace the productivity, nor do I think it makes sense to create another roster hole at this point of the offseason.
who cares? They have the room to do so this year, unless you think there was some massive cap-altering move coming later this offseason, which I do not.
His cap figure next year is even less, and the team has even more cap room.
It makes no sense at all, other than the fact that our GM is a glorified scout who loves his draft picks.