Polk is a beast and will definitely be missed. He's a complete back. He excels between the tackles, on the edge, at pass blocking and catching balls out of the backfield... He fumbled twice in his collegiate career. The only thing he lacks is pure top-end speed. He should definitely fit in well in the NFL as he can do just about anything, and has great YAC.
Luckily, UW has some talent in the 2 and 3 deeps at the position. I would expect continued good production overall from the RB position with a more experienced OL, QB, TE and WRs (younger but far more talented in the underclassmen who now have another year experience). It won't be the same without CP, but there is young talent at the position still.
Overall I think UW's offense will be a little more potent next year since most starters return with another year of experience under their belt, expecially if Keith Price can stay healthy. If Price isn't healthy then all bets are off. We lose a little experience but for the most part (exception of Polk), the incoming young players have a lot more upside than the departing seniors.
The biggest question is whether the defense can be mediocre or better...