I hate these fucking people.
Look. Lifetime of centuries? That means that if they are even 1/10th as fertile as humans, their eternal youth means they can still have plenty ofbkids.
Ten a century from a dedicated couple? Not bad. Considering that the mother and father will still be around centuries later.
Eldar growth could very well be glacial in pace, but it is also glacial in durability.
Humans have on average 30 years of prime fertility for women, and they begin degrading in physical capacity at the later stage. So female humans have to choose between children or career. Many simply choose one or two kids.
But longer lived races, from Vulcans to Krogan, do not have this choice forced on them. They can have a dozen kids and still have time to enjoy life without being decaying nearcorpses.
And for the Eldar, if they have 12 kids over 200 years, thats still a net gain even though it took forever. Meanwhile a human could pop those out in a mere 16 years or less.
But if an Eldar family loses half of there children to war....thats decades and decades of lost investment in just making them, much less raising them. Without extreme technology, and having a decent sized percentage of their populace engaged in War, they will constantly be on a longterm losing streak in numbers. The Eldar are breaking under constant, severe pressure without the tools to deal with it. Eventually, the rate of losses will trigger a death spiral and they wont be able to maintain what they have and will constantly be in retreat.
The Eldar can only lose ten soldiers to the Imperiums 500. But they are losing 30.
So even with a population in the hundreds of billions, they can still be screwed.
Imagine if Europe couldnt take in immigrant and wouldnt increase native birth rate. Ultimately they would simply die out, even though it would take a few centuries. Same thing with the Eldar. Except played out with a race capable of touching a millenia.