I agree with their assessment.
Smart is in the bottom 20% of total shooting% and turnover rate% in the NBA, he hurts our offense almost as much as he helps our defense. He is good enough to be a rotation player and sometimes finisher, but that's it. He could be a late bloomer, but on a bunch of offensive benchmarks, he has had little progress other than on assist% and this year he's below his career averages on PER, total shooting%, and turnover% and none of his career averages are great. He has also been surpassed as a two-way player by Rozier.
On the defensive side, he is all-NBA but similar all-NBA defenders Roberson of OKC and Beverly of the LAC make on average $10MM and $6MM, respectively, on contracts they signed in 2017 and 2015, respectively. They are both all-NBA defenders and both offensively challenged. You're not getting Smart (or Beverly in 2018) for $6MM, but getting much past $10MM is betting against 4 years experience that Smart's going to improve much offensively.