Everybody staying healthy is a pipedream. Did you forget that Hoerner had surgery that may keep him out the first part of the season?
Shaw is going to get a ton of chances in ST to prove his worth. And he's likely to line himself up as a the primary backup to 2B, 3B, and SS. He may start the season with the Cubs depending on Hoerner's recovery, but once Hoerner is good to go it probably makes sense to put Shaw back in AAA for his development. But I'd bet dollars to donuts that any prolonged injury to Swanson, Hoerner, or Paredes ends up with Shaw getting the nod.
I'd guess Shaw logs north of 60 games with the Cubs this year, and is poised to take over a spot next year.
And to touch on a point you've made to other people -- I don't know why you're so concerned about offensive runs versus defensive runs. There are arguments about the proper roster construction to be made in the playoffs, but for the regular season runs are basically runs. There's no point of diminishing returns on defense during the regular season. Remember 2016? We had the best defense in baseball.