I agree, there's no way to know he'll ever be good. Just like in football, sometimes draft picks just aren't that good and you can tell by year 2 (T Richardson). Heck, Kendall Marshall is in the D League now and Austin Rivers is only hanging on because of his famous dad.
Given the market for Derrick Williams (which netted an overpaid, one dimensional defensive stopper) may mean the asking price won't be too crazy.
If you don't like his game, then why screw with a good thing? If the Spurs stunk and Cavs were looking to contend, that's a completely different story.