I think the Twins would trade one of Kepler or Rosario once AK is ready.
Rosario isn't a great fielder, but he's far too competent to waste at DH- where his bat wouldn't be great to start with.
I think this is more reputation based and the fact that defensive stats penalize him when Buxton is on the field. He's got + range, though he does make the occasional bone headed throw... His bat has more than made up for it. You're right though, if Kirilloff progresses similarly in 2019 than he did in 2018, I don't see a scenario where he's not the starting RF in 2020. And that means Rosario, Cave, or Kepler is traded/non-tendered. If all are doing well, someone will get a nice return, as all of them can play CF.