Smart is 2.5 years in and still a terrible shooter. It seems pretty likely he will never develop the shot based on where he is currently at. The fact that he is ranked ahead of someone like Tobias Harris shows how good Smart's defense is and how much respect he gets because of it.
Why does it seem pretty likely he will never develop the shot? You just said he's only 2.5 years in. Is that a long time? Isn't he still further than 2.5 years away from his prime?
TB12 taught me to no longer constrain my thinking to "well, the statistics show it's unlikely" or "this guy can't improve".
I never understand the pessimistic view point while watching sports. Seems way less fun.
Because Smart was a bad shooter in college also and hasn't shown any real ability to be a good shooter at the professional level. And this isn't a case of a guy not getting consistent playing time or barely shooting from 3 where it is hard to find a groove. I can't think of a single guy that all of a sudden had a shooting breakthrough that people on here expect Smart to magically have.
Smart just needs to stop shooting from outside. In fact, that would greatly improve his offensive output as you can at least be good (if not great) offensive player without having three point range, but you can't shoot the ball as much as Smart does and do it.
I'm not sure what passes your "real ability" threshold, but other posters have mentioned his recent free throw shooting streak. In addition, he has a couple game winning shots as a Celtic from deep. I use the eye test to judge in this case. From a mechanics perspective, he has a better looking shot than guys like Josh Jackson and Lonzo Ball. I'd say Marcus has close to average mechanics for a guard.
Yes, he's been given a ton of minutes, and yes, he's missed a ton of shots, but his defense is good enough to keep him on the court. I also consider him a developing player. If he doesn't progress much, he'll be a Tony Allen type. If he does develop an offensive game, he'll be an all star.
I see no reason not to root for the latter.