If the C's think Smart has the potential to do that, it seems odd to me that their philosophy for his offensive development is shooter or bust.
I think this overstates how things have gone. You're overlooking the fact that he was a relatively young rookie.
The Celts gave Smart a limited role in the offense, probably for his own good because he was not and probably still is not a great ball-handler or decisionmaker. They gave him a limited role in the offense and let him focus mostly on doing one thing well -- getting open for, and taking, three pointers.
That also happened to be an area in which he was fairly weak coming in, an area he'd need to improve significantly in order to open up other areas of his game -- attacking the basket, scoring from mid-range, drive-and-kick -- that perhaps didn't need as much work.
Food for thought: if you're a bad three point shooter, you're not going to be allowed to take that many shots from beyond the arc.
Probably so. I imagine that in practice, both Smart and Sullinger have looked pretty good from deep. For Smart, consistency in his form seems to be the biggest problem. For Sullinger, it's his conditioning -- a lot easier to hit those shots when you're not winded and you haven't been running up and down the court for an hour.
Throw in Avery Bradley, who always looked like he could shoot but didn't start to actually do it consistently until his fourth season.
Smart's actually ahead of Bradley in that regard.
Typically, players with the kind of shot selection from deep that Smart exhibited in his first season either end up riding the bench a lot or turn into high volume scorers. The fact that Smart was playing pretty major minutes and starting by the end of his rookie season bodes well for which direction his career will take him.