It's not like he truly earned playing time in the majors. He hit pretty well in AAA, with a 119 RC+. But the fact of the matter is, so do plenty of people you can pick up off the waiver wire at any given point of the season (Andrew Stevenson?). Keirsey could have also earned time by being a great defender, unfortunately his glove is reportedly...serviceable.
I feel bad for the guy, but it's not like he's getting screwed. Last season, I would agree that the Twins really screwed up by refusing to DFA Manuel Margot and giving him a chance to play. But not because he was good, but because Margot should have been DFAd in April as soon as Rocco realized he could not play a decent CF.
But as for his career, Keirsey had to either hit more or field better in order to become a proper major leaguer. That's just his sad fate. AAAA player.