It depends on your timeline. If, as you say, the illithids have been on Athas for a long time, then natural selection would have it’s effect. In my example, if the campaign starts at Free Year 10, the illithids have been on Athas a mere 4 years.
You’ve given me an idea, however. Without the elder brain to nurture the strong tadpoles and absorb the weak tadpoles, there is no way of ensuring quality illithid breeding. Therefore, 9/10 illithids born on Athas can be like the savage yaggols from the Dragonlance setting, and some failing to even full go through ceremorphosis, becoming half-illithids (as in the 3.5e template).