Speaking as someone who lives in Massachusetts, I think that Charlie Baker would defeat Ed Markey if he chose to run. Nobody knows who Markey is, but everyone loves Baker.
Parties running popular former governors for the senate in states that are extremely partisan against them don't work as well as you'd think.
Ex: Phil Bredesen, Linda Lingle, Mitt Romney 1994.
Baker will still be the sitting governor in 2020, though I agree that he won't win if he runs for Senate, especially with Trump at the top of the ballot.
Weld was more popular than Baker is in 1996, and
got absolutely smoked by John Kerry even as the incumbent governor, and only two years removed from winning the biggest landslide in Massachusetts history.
I hope the Republicans nominate Baker, so they pour money into a Safe D state and still lose by 12.