The Eldar definitely do not evolve. They're an artificial race, like the Orks and Necrons before them, incapable of changing even over a million years by the Old One's design.
And humanity is definitely evolving to become more Emperor-like. Not only was a test acceleration of this process the stated goal of the Primarch Project, but even with all of the prejudice against psykers they've only managed to become more common. MIND BULLETS is just too good of a trait to be bred out, even when balanced against daemonic attention. Back in the Emperor's day there were only a few psykers around, and their collective suicide deprived humanity of the trait for thousands of years. Now, pretty much every world in the Imperium has both sanctioned and wild psykers on it, with more in every other human settlement in the Galaxy.
Further more, the evolutionary advantage of witch genes starts to assert itself long before a single psyker discipline can be harnessed, through what is called luck but is actually subtle reality bending, but is luck because that's what luck is and has always been. Between that and latent witch genes, even if the Imperium culled every single psyker they could find it still wouldn't stop the rising tide of human transition to a fully psychic species.
Of course, without the Emperor or Magnus around to help guide the process, getting the kind of secondary conditions like the Eldar possess to have the power without going utterly mad may prove...difficult.
That said, it actually is a point against the Tau as well. There aren't lucky Tau because luck is a warp phenomenon, and in 40k, sometimes luck is all you can rely on.