Fit matters. That's what the Cavs are now learning the hard way. Mitchell fits, but now... Garland doesn't. And that's something of a problem, as they just gave him the 5 year max and stupidly anointed him the face of the franchise. Garland is quickly becoming the elephant in the room.
I'm not saying they shouldn't have done the trade. Don is much better than Garland. But they're way too similar, and something has to give before long. That will be very uncomfortable for everyone.
Of course, it's not a given that Mitchell even stays in Cleveland once his deal is up, but if the Cavs have become "his" team in the meantime, Garland may not want to look like a plan B after Don bolts. He may ask for a trade too (if he hasn't earlier). At that point the Cavs are ****ed.
So, we definitely shouldn't be hoarding guys like Collins before thinking long and hard how they're going to fit.