Putting Paxton in the bullpen isn't a one way street. Given that he hasn't cracked 100 IP in either of the last two seasons, they probably have him tabbed for 120-130 IP total this year. Absent a trip to the DL, if he's starting from day one, he's either not pitching the last 6-8 weeks of the season or he's doing it from the bullpen. Why not get those bullpen innings at the beginning of the season (when the club really needs them) instead of the end of the season?
Stick him in the pen, tell him to focus on his two best pitches (FB/CB), let him go 2-3 innngs per outing and see where you are at in a month or two. Maybe he goes thermonuclear in that role and the club has a bona-fide relief ace on their hands. Maybe not and once the starters get stretched out and the weather warms up, you ship him to Tacoma to get his innings in. Regardless, it's not like a month or two in the bullpen at the beginning of the season pegs him as a reliever for the rest of his career.