That's one way to look at it. Another is to say that there are probably a lot of guys who could be just fine given an opportunity. Slegers might be one of those guys. His ceiling is probably a #4/5 starter in the big leagues, but he's really good in AAA. He could go to the Marlins or something and pitch in their starting rotation (maybe as their #3/4) for 4-5 years.With the Twins, he's kind of a #6/7 right now.
Slegers is a great example of what I'm talking about. I happen to believe he could settle in as a workhorse in a good rotation. But, the lack of strikeouts is a warning flag for many, and maybe I'm foolish to believe in him. It's that difference of opinion that might give him the Marlins opportunity you speak of, if the Marlins are more interested in results than form. But no way were the Twins investing minor league starts in him because they were hoping he reaches a ceiling of a #4; the opportunity cost is too great through the years as he's coming up the ranks.
These rotation numbers are so arbitrary and roster-dependent as to make them meaningless, except as "potential #1/ace", "potential #2/#3", or "make him a reliever before it's too late". Someone who's a AAAA guy flunked out of whatever ceiling people thought he had.