Oh, Eldar were fucking scary back in the day. Don't forget that they used to be a real Empire, with Craftworlds as their goddamned merchant ships, that only died because of the unforeseen consequences of being so successful they were able to descend into the depths of hedonism. When there were actual wars to fight? Consider how skilled a single Craftworld Eldar is, except a career soldier from a long and glorious tradition of soldiering, rather than a member of a dying race trapped upon their ships who struggle to emulate their gods and history in mourning for their dead fellows. Imagine them equipped with true military-level technology, and that the Craftworld Eldar stuff is, as far as it seems likely, the civilian/merchant defense gear. Consider how many more Eldar there were, all with equipment on that high of a level. Their equipment is all small and energy efficient because that's cheaper to have on board merchant vessels. The actually somewhat bulky shit that was given to the real warriors is probably as lost to them as the solid-light ships are to humans.
Yeah, they're still not uberperfect, but there's a reason they're pricks about being superior, just like there's a reason Humans do much the same, and are likewise watching their Empire collapse, albeit slowly and with great reluctance this time around.
Thing is, Empires only last until they don't. Probability means that as time goes on, the likelihood of it collapsing approaches 1.