Paxton might never top the ~160 innings he threw this year. He's damn talented but he's hardly an Ace.
Clearly, there is a lot of injury concern baked into this deal.
I also think Swanson is being disregarded unfairly. The quality pitching depth that the M's just got, cumulatively, is going to be way more valuable to them than Paxton. There is a decent chance Swanson is actually the better MLBer between him and Sheffield IMO.