Marcus Smart probably has a solid Tony-Allen-like career ahead of him. As a minimum. Perhaps slightly better.
The fact we're looking at him as a franchise cornerstone is slightly depressing.
Slight better on offense, slightly worse on defense.
I would say MUCH better on offense, slightly worse on defense.
Tony Allen never really had any offensive talent / skill whatsoever aside form the ability to drive and get to the basket.
Smart has three point range, can pass, can handle the ball. Even if he never developed at all as a player, that's already a huge distinction over Tony Allen...who was really never anything more than a (really good) defensive role player.
That seems like a reach. I agree that Smart is more likely than not to become a better offensive player, but that isn't supported by stats at this point.
TA's rookie year was actually better than Smart's in more aspects than not, and he generally improved over time (glaring exception: 3P%). He played far fewer mins which could arguably be a confound, but because his stats remained mostly consistent throughout his career and gradually improved a bit, it seems like a fair comparison to make.
Here's the rookie comparison (Per 36 comp):
Smart: MPG=27, FG%=.37, 3P%=.33, FTA=2.5, FT%=.65, TRB=4.1, AST=4.1, STL=2, PTS=10
Allen: MPG=16, FG%=.48, 3P%=.39, FTA=4.3, FT%=.74, TRB=6.3, AST=1.8, STL=2, PTS=14
Given Allen's rookie year (albeit 1 attempt per 36), Smart's college percentage, and SL shooting, I wouldn't say Smart definitively has 3P range yet. Seems an argument can be made that he has been reliant upon 3s because he's struggling to get to the rim.
Allen was a mediocre ball-handler, but he could get to the basket. So while Smart may be more adept at getting to where he wants as a playmaker, if he's limited the advantage is significant but not huge.
Allen also excelled in transition, haven't seen that much from Smart.
Again, it's too early to tell. Just don't appear to have enough evidence yet to have any definitive answers re: Smart. I hope for the best, honestly, and I do think he'll significantly improve.