I am not going to go into the whole ‘Forsaken didn’t get enough attention in SL’ because that is just a waste of time tangent discussion. And I think we can both agree it wont go anywhere good. Instead, let’s look at the current situation and the future.
Let’s assume that the Forsaken can raise more, reliably and reasonably easily. That is a logical assumption. Between their own research, what they learned from the Valkyr, and what they would have learned in Shadowlands we can reasonably assume they would have that particular problem at least mostly resolved.
Thing is, that was not the only thing limiting their population growth.
The biggest issue:
They also need a supply of willing corpses. Raising people who did not want to be raised would just cause them more problems. It would cause splinters among them, rebellions, rage, etc. The only reason the scourge could grow like it did was because the raised were mind controlled, they had no choice. That is not something Forsaken can, or will, do. Leaving the only supply of new recruits to be those that choose to become Forsaken. While there will be some, there wont be a ton. Not enough to significantly increase the population.
I think the best way to think of the Forsaken replenishment is that it is around the attrition level.
There would be some who feared death so much they would choose to become undead, though the concrete knowledge of the afterlife would limit the number that would feel that way. And there would be some who would choose to be raised so they could continue fighting for the word. But on the flip side, there would be Forsaken just destroyed in one battle or another. Also there would be some Forsaken who just decide they are done and no longer want the pain of their existence. Those likely are at least close to balancing.
Couple that with the low population they are starting with and their population will never be very large.
Intact? No. Destroyed? Also no.
It is safe to say the Horde lost ground and ended up in a weaker/worse position post 4th war than they were before it. But that doesn’t mean completely wiped out.
Some amount of refugees fleeing does not equal the entire population fled.