There are a few options. One is to hold on to the All-Star and put a serious team around him that he feels has a chance to win from the get-go in 2019, and hope you can convince him to re-sign when he hits free agency following the 2019 season … but then the Colorado Rockies risk him walking and receiving nothing.
Option two is trading him this offseason and just ripping the Band-Aid off without letting it fester, like Baltimore did with Manny Machado. You trade Arenado and, in turn, receive the most you possibly can.