As mentioned elsewhere, he has played third base more than anywhere else in his professional career. And there are plenty of ways to give players a look at a defensive position, especially an infield position, without spotting him 60-80 starts in the majors. You could knock out the number of opportunities he would get in that stretch in about an hour and a half of pre-game work.
The idea that the Cubs, with their second baseman, shortstop, left fielder, and right fielder all under contract for multiple years, would just now, 575 games into Morel's professional career, be like 'oh man, we never really thought about him as a third baseman, should we try to make that work?' is pretty absurd. Similar to all the comments about 'they probably just wanted him to focus on his offense', as if Morel, by all accounts an elite 24 year old athlete, spent all 5-6 hours of pre-game in the batting cage or was somehow too tired to field groundballs after all the exhausting 'focusing on offense' he was doing.
Could the Cubs be wrong? Sure. But I'm pretty sure this isn't some novel idea that hasn't previously been considered.