Mondesi has a career .282 OBP. His defense will always be valuable, but he's not a cornerstone you build around unless he can greatly improved as a hitter.
In the most recent Tigstown podcast Mark talked about the very real possibility of the Tigers having to trade some of their pitchers 3-4 years down the road because the timeline might not matchup for when the Tigers system starts to produce position players. Soler and Dozier may be legit, but the Royals are terrible with them already and they are already 27 and using up service time. By the time they may actually start producing other players those guys will be 31+ and either be gone already or approaching free agency. They simply do not lineup with the Royals next contention timeline at all. The only value those guys provide to the Royals going forward is as trade pieces.