Idk, I don't think the Eldar are limited in such a way as to be inferior to humanity in the long run. While humanity is pretty much confirmed as to being the wildcard of the galaxy due to having a shitton of psyker potential that is developing at a pretty fast rate, the Eldar are still kind of developing in their own way. The eldar have probably reached the pinnacle of what physicial evolution allows, but haven't been able to transition into fully spiritual beings like the old ones did because, you know, the spiritual world became chaos and there's an entire chaos god dedicated to capturing their souls, so transcending the materium is probably not such a great idea at the moment. The Eldar are probably completely aware of that, too.
They could've totally followed into the Old Ones' steps if they didn't fall into hedonism and general degradation, and while the surviving non-dark eldar kinda got better later (spiritualy, at least), they probably stunted their evolution intentionaly so they don't become even more vulnerable to slaanesh. This is why they're now relying on soul stones and the whole Ynnead infinity circuit thing, so they can safely store their souls in the materium, power up and then, perhaps, when its safe enough, pick up the pace and transition into fully spiritual beings.
Humanity, on the other hand, while certainly being vulnerable to chaos, is also kind of scary to chaos. Its pretty much stablished that the fates of chaos and humanity are intertwined, and that the Emperor's real goal was to usher a new age for humanity, to bring it to a whole new evolutionary level while at the same time protecting it from chaos, to the point it wouldn't need him anymore. He's kind of still trying to do it, just in a less direct manner.
The Tau are weird. Due to their doctrine, their government spreads and gains power pretty fast, and their technological progress is clearly at a faster rate then anyone else in the galaxy, but their spiritual progress is almost null. They don't have psyker potential (so far) and their souls are really tiny in comparison to pretty much anyone but the necrons, who (mostly) don't have souls (altough the ones that DO have souls are pretty powerful). The reason chaos doesn't care about them is simply because there's bigger fish on the table. I mean, would you care to eat bread crumbs when there's steak and fancy lobster dishes readily available to you?