It's also worth noting that the Eldar only have to be super careful because there's a chaos god waiting specifically for them, who has a unique bond with them. Pre-Slaanesh, they were a race of latent psykers that didn't live in constant fear of daemonic possession.
They were a race of psykers before the fall as well, seing as it were their seers who saved the race by predicting the fall and warning enough people about it.