I'm not expecting Santana to be more than he has shown to be the past few seasons, which would still be a big upgrade at 1B, and he wouldn't require a platoon mate on the roster to have competent offensive production from a corner IF position. plus he's still a quality defender over there.
The past few seasons, it seems like the Brewers have been chasing their tails trying to set up platoon options at 1B and 3B plus DH, often taking up 3-4 or even 5 roster spots and wind up getting below average offensive production to show for it from all 3 of those everyday lineup spots. To me, there's value in plugging Santana in as the primary 1B and dedicating 1 roster spot to that position, then rolling the dice with Black at 3rd and signing a veteran DH to give your roster flexibility/depth elsewhere. DH, IMO, should be a bigger priority in free agency for a team like the Brewers than 1B...if their model is defensive versatility to build the position player roster, I'd be much more ok with that if they would view the DH as more than an afterthought/bench role when Yelich or Contreras isn't occupying that spot in the lineup. Even if the Brewers hold onto Burnes to start the season instead of trading him (and I hope that is the case), they have plenty of budget available to sign Santana and one of the more sought after DHs...particularly if they let Black open the season at 3B making league minimum.