You say that as if it's a large number. That's a varus auto attack range. That is incredibly tiny.
Enemies that aren't near allies should get the isolation, those who aren't shouldnt, no way to weasle it in there because you want extra damage
Seeing as without isolated Q he does really low damage, yeah, it is.
Normal Q: 170(+120% Bonus AD)
Isolated Q: 221(+156% Bonus AD)
Evolved Isolated Q: 221 (+10 x Kha's level)(+260% Bonus AD)
One normal Q has the same ratio as his W + E combined, which makes getting isolated Q's off REQUIRED to have any kill potential. Nerfing his range even more just makes Kha'Zix literally useless as an assassin (most assassins already can you kill if your team isn't around, and don't require you to be standing away from any single allied thing to do it).