We got to within one game of the World Series in part because of Burnes' contributions in relief in 2018. And then, without the failure that was his 2019 season, Burnes would never have reworked his arsenal, particularly the cutter, which has turned him into one of the game's very best pitchers. So, not sure that logic entirely works out.
These guys aren't machines. Not that often is top prospect going to come up and "hit the ground running", as you say. There's a developmental process that can only occur in the majors, and the Brewers seemingly recognize that. Turang has been in AAA for a year and a half now. There's nothing left for him to do there. In Jace Peterson's role, he would get regular at-bats, especially with Wong likely sitting against LHP, without the immediate pressure of having to hold down a starting spot and produce. It's the optimal situation, if you ask me, and it certainly wouldn't be a "waste" of his service time.