The abberation does not equal the mean. IT is above average on offense thanks to his speed and quickness, The thing is somehow that speed disappears on defense and he covers guys like his shoes are made of concrete. While he may have the strength to counter being posted up at times, most NBA teams aren't going to send their 1s into the post since that (a) turns their facilitator into an isolation scorer (tall heroball) and (b) most PGs lack the experience and skills required of good player.
As just about everyone has been saying for god only knows how long, IT's lack of length and quickness is an absolute drag out on the perimeter, where his man often gets to camp out wide open along the arc because, for whatever reason, he forgets to cover his guy (fixable but little progress has been made in the last three years) and, when he does notice him, he lacks the quickness (I have no explanation for how this is possible since he'll routinely beat the same guy on the other end of the court) and the length (he's scouting combine 5'9", meaning he's probably 5'7.5" on a good day) to get to his open man in time to affect the shot.
More disconcerting than that is IT's lack of tenacity when it comes to defending the screen game. If a big man sets even the most rudimentary of screens, IT will either completely sag off his guy rather than try to fight through (another product of slow defensive feet?) or initiate a switch with his big man, which means that there are a lot of mismatches in which a slow-footed big man is forced to defend a quick-footed guard, which, unless that big man is Anthony Davis or Serge Ibaka or another laterally gifted player, is usually going to end badly.
Much has already been made of IT7s superheroball offensive tendancies but it's really IT's defensive flaws that make him expendable in my opinion. It's not just that he's short. I wouldn't want a 6'2" Isaiah Thomas either. Now if Isaiah Thomas became Isiah Thomas, who wasn't exactly tall himself, I'd be more than fine with that, just so long as we didn't let him be GM.