system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40K] For a bunch of immortals, the Eldar don't seem particularly good at the long game.

My base assumption is that the Eldar's reproduction rate is basically near 0 right now. Every other race outbreeds them except for maybe the Necrons (sub question, how do Necron's reproduce, if they do?). In order for the Eldar to hold on, they need to win by vastly more than they lose.

The Eldar have, on some occasions, tried to help The Imperium. They've tried to warn the Primarchs/Emperor, they've allied with the Blood Ravens briefly. The problem is, the Eldar, 9 times out of 10, will betray you. It sets a precedent. I realize that a human life is worth less than an insect, perhaps even a bacterium to the Eldar, but there is a reason for cooperation and mutual support. If you show yourself to be reliable 9 times out of 10, you'll get a lot more mileage out of your mutual interests. Maybe that's not a skill the Eldar ever learned in their thousand year life-spans, but it seems like the Tau and Imperium picked those traits up in a much shorter period.
gpt
You act like it's the Eldar who are to blame. The Eldar race who have lost everything and are merely trying to scrape together what's left of the Galaxy with their dwindling numbers and ages of wisdom.

Versus the Imperium, who hates anything that isn't human (and even many humans) and won't listen to what an alien has to say until it's tried killing it about a hundred times.

> (sub question, how do Necron's reproduce, if they do?)

They don't. They are basically legions of metallic undead.