It's a legitimate concern.
If you look through league history, it is difficult to find examples of score-first guards under 6 feet whose careers lasted very far beyond age 30.
Of course, that's a small sample size, and many of the players you do find in the sample played in a league with a different ruleset. Perhaps waterbug guards will be able to last longer in today's league without handchecking and more spacing.
JJ Barea is 31 already and he's still playing really well. He's pretty small, too.
Wait who exactly are we talking about here? What players scored over 20 a game, made an all-star game and where under 6 foot?
OK, well, there are, as you know, very few players who fit that specific qualification.
But here's a list of guys below 6 foot who scored a lot of points, and a general estimate of when they stopped playing a big role.
Dana Barros, 5'11'', scored 20 ppg his age 27 season, fell off significantly after his age 30 season.
Damon Stoudamire, 5'10'', averaged around 20 ppg his first few seasons (age 22-24), had a pretty good season at age 31, then fell off after that.
Michael Adams, 5'10'', was an All-Star his age 29 season scoring 18 ppg, then began to decline and fell off big time after age 31.
Terrell Brandon, 5'11'', two time All-Star at age 25 and 26 scoring 19 ppg, fell off toward age 30 and didn't play after age 31
Calvin Murphy, 5'9'', All-Star at age 30 scoring 20 ppg, had another 20 ppg season at 31 followed by a decent 16.7 ppg season at 32, then fell off, not playing beyond age 34.
Nate Robinson, 5'9'', career sixth man who peaked at 17 ppg at age 24, had another decent season at 28 for the Bulls, didn't play meaningfully beyond age 30
I'd say Calvin Murphy is a nice best case scenario. Top-option scoring through age 31, then a few more reasonably productive seasons after that.
Isaiah turned 27 this season. He'll be 28 next February.
Personally I think it's reasonable to hope for another 3 or 4 seasons of 18-20 ppg level production from Isaiah if he can avoid major injuries (i.e. age 30-31), and then another 2-3 seasons of solid backup type production (e.g. Barea) after that.