Comparing Smart to Iggy just shows how insanely off people are with evaluating Smart.
First of all, Iggy posted moderate-to-good stats for the majority of his career while still being a great intangibles guy. Secondly, he took nowhere near the number of dumb shots and was never a ball-stopper on offense, which is part of the reason why he fit so seamlessly into the Warriors' offense. Lastly, Iggy could do a little bit of everything on offense and most importantly finish at the rim with consistency.
In no way, shape, or form does Marcus do any of the things I just mentioned. Smart is an historic trainwreck on offense, and he'd have to be Kawhi Leonard-levels of good on defense in order to justify putting up with that sort of offensive ineptitude. Unfortunately, he's not. Quite frankly, the team's defense is based on the scheme and getting everyone to buy in and exert effort on that end - it's not being made or broken by one guy. That's why we were able to let Bradley go yet still function at a high level.
Unfortunately, it doesn't work like that on offense. You can't hide a guy on offense in the modern NBA; if he can't or won't shoot, his presence in the offense just shrinks the court since nobody will defend him at the 3 point line (which is particularly evident in the playoffs when teams have more time to implement schemes like that). And that's the very best outcome... the more realistic and historical outcome is that Smart will just jack up tons of bad shots and stop the ball from moving, completely killing any semblance of a functioning offense.
Finally, if you think the way Smart has been playing is bad, just wait until you see how terrible he'll be once he's gotten his contract. I guarantee you it will make Evan Turner's contract look like a steal by comparison.