bulbaquil
...is not Sjadbek, he just runs him.
I really think it might partially depend on the Altmeri life cycle and the relative populations.
If Altmer are like D&D elves, not reaching sufficient physical maturity to fight until 100 (or, heck, even 30), the significantly faster maturation rate of the Nords/Redguards/etc. (assuming it's analogous to humans') means that it takes much less time for the Nords/Redguards/etc. to "replace" a fallen warrior in the population than for the Thalmor. This is probably why they're trying to work subterfuge with the Skyrim civil war (to "reset the humans' timer," so to speak, as they try to replenish their own forces) and why their own dossiers state that either a Stormcloak or an Imperial victory would be detrimental to their plans.
If Altmer are like D&D elves, not reaching sufficient physical maturity to fight until 100 (or, heck, even 30), the significantly faster maturation rate of the Nords/Redguards/etc. (assuming it's analogous to humans') means that it takes much less time for the Nords/Redguards/etc. to "replace" a fallen warrior in the population than for the Thalmor. This is probably why they're trying to work subterfuge with the Skyrim civil war (to "reset the humans' timer," so to speak, as they try to replenish their own forces) and why their own dossiers state that either a Stormcloak or an Imperial victory would be detrimental to their plans.