OK, check this out.
I went looking for players at guard who registered at least one season with 20+ minutes per game, 20%+ usage rate, and 16+ points per 36 minutes, during the 3 point era, at age 30 or older.
That's how I loosely define a "scoring" guard, i.e. not including guys who played predominantly as a facilitator. I agree with others that IT's primary value to the Celts is as a scorer, so what we really care about is the prognosis for his ability to continue bringing that value to the table.
When you look at players who were listed at 5'11'' or shorter, only four names come up:*
- Calvin Murphy, who had 3 such seasons
- Chucky Atkins, Earl Boykins, and Damon Stoudamire, who each had one such season.
*Terrell Brandon had one season with at least 15+ points per 36 minutes.
When you look at players at guard or guard-forward listed at 6'0'' - 6'4'' (i.e. average guards, excluding more prototypical wing types like Kobe and Vince Carter), with otherwise the same criteria:
There are 61 different players who meet this criteria in at least a single season.
Nine players had at least five seasons meeting this criteria (ie five seasons age 30 or older).
Sam Cassell leads the list with 9 (!), followed by Ricky Pierce, Chauncey Billups, Steve Nash, Jeff Hornacek, Allen Iverson, Gary Payton, Jason Terry, and Dwayne Wade (who will probably add a few more seasons before his career is done).
To me, that's fairly conclusive that bigger scoring guards last considerably longer than smaller ones, as a general rule.
That doesn't guarantee that Isaiah won't continue scoring at a good rate until he's 34 or 35. It just means there are very few examples of guys who have done anything like that. In fact, I think Calvin Murphy might be the only one.