To offer friendly counterpoints to the Lightforged-population theories, my own headcanon is that the greatest strength of the Lightforged was (key word: was, to be explained) that they were really, really, really freaking resilient. Thanks to themselves, and their healers especially, directly tapping into the power of Xe’ra, they were able to recover from grievous injuries that no healer of Azeroth would be able to heal (unless another deity-level being was giving them a boost, but that tends to be rare). If a Lightforged managed to keep themself in one piece, even after being skewered by a felguard or blasted by a pit lord, they could recover in time. Therefore the Lightforged managed to maintain a reasonable population within the Xenedar as they fought across the cosmos, as the beacon technology combined with their hit and run tactics (rather than fighting needlessly to hold any one ground…which, unfortunately, meant they would never commit fully to save an in-progress Legion invasion, as the outcome was already decided, even if it took the Legion centuries to conquer yet another world), meant that it was relatively easy for Lightforged to retreat with their injured comrades. And thus while the veteran Lightforged are certainly among the greatest of paladins and other classes, they’re still not superhuman (superdraenei?) more any other experienced mortal champion. Millennia old or not, there’s still hard limits of how fast a mortal body can move or how much magical power they can produce from their bodies. And once Xe’ra was weakened by casting her Heart to Azeroth, the Lightforged were no longer able to draw such miraculous healing Light from her, and their causalities began to mount rapidly over the course of their initial campaign to weaken Argus in preparation for the arrival of Azeroth’s champions.
And so, while the Lightforged have never let the deaths of their brethren falter them, it is a solemn occasion to note and pay respect of, at least when they have the luxury of time to mourn. Unfortunately, the war on Azeroth has reminded them all the more of their mortality as more of their kin fall even after the greatest and impossible of victories at Argus, and reinforced the regrettable need to forge more of their Exodar cousins in Light. One day, such a rite will no longer be necessary. Or at least that is what they pray for.
(Also, I too was rolling with the notion that Lightforged require less of the mortal necessities (though I went with less, not removed entirely) of food, water, and sleep. They are also more resistant to fatigue and the effects of extreme heat or cold (but limits do still exist, to be sure), to say nothing of their nigh-unbreakable morale. The power of the Light could cleanse fel tainted water from the planets they were on and meat from the beasts that managed to survive on Legion worlds, but obviously harvesting such things is a challenge nonetheless. Ultimately, the Light sustains, or at least for a decent amount of time. Basically, reinforcing the above point that the Lightforge’s greatest (and conveniently not super overpowered) strength is their resilience able to fight an endless war.