Grimm is at the bottom of the bullpen list on purpose. With the possible exception of Wilson (if you judge him based only on his time with the Cubs) Grimm had the worst 2017 out of everyone else who is projected to be in the Cubs 2018 bullpen.
There are severa other relief pitchers likely begin the season at Triple-A but could be called up anytime in 2018. This group includes Dillon Maples, Dario Álvarez, Cory Mazzoni and Randy Rosario.
Pushing to the front
So there will be several other pitchers pushing for a place in the Cubs bullpen. Last season when Grimm struggled some, he could and was sent down to Triple-A. This year that is not a possibility without losing him.
Despite getting a contract through arbitration if Grimm is cut during spring training he only gets a prorated portion of his salary. Arbitration contracts do not become guaranteed for the year until after spring training. So financially, cutting him during spring training is a realistic option.
While Joe Maddon prefers to carry eight pitchers in the bullpen, seven is more common. The positional versatility of several of the Cubs hitters makes this more of a realistic option for the Cubs than most other teams. But the Cubs don’t usually have an eight-man bullpen during the entire season.