There's nothing the Mariners like more than preparing for whatever year they are not currently playing in. At least they let Kyle Lewis back in the field after DHing him during most of the year to try to keep him healthy. He started off on fire in the field, then they DHed him for a month where he was then terrible (.710 OPS as a DH, .855 as a CF) before returning him to the field.
I'd say it was precautionary and wise, but it's the Mariners and they really have no idea what either of those terms mean.
This'll make 7 HOF-level talents we've wasted, right? Griffey, A-Rod, Randy, Edgar, Ichiro, Felix and Cano? That doesn't include Moyer, Cruz, Bone, Olerud or other Hall Of The Very Good players. I'm trying to think of other teams with that much talent in 3-decade period who accomplished as little, but I'm coming up short right now.
But don't worry, Leake for 2018 is a terrifically moderate plan.