Both can be true in this case, they have the population to sustain it, however the economy is on borrowed time, just like any war. Ukraine is attacking the economy. So the war weariness is increasing. The population has a constant supply of new births however unless you maintain the economy it won't grow on its own, unless you capture more land or production facilities.

It's how the us effectively lost the Vietnam war, it was a political loss, the war weariness caught up on them. Yes they won ever battle, yes they still had the manufacturing output, however they had to pull out. The same thing is happening right now in the us-iran war, and probably why troops haven't engaged in a land invasion yet.

I do get what you are thinking, it sounds like fascist propaganda that the enemy is weak and strong at the same time, but I think it falls apart a bit once propaganda becomes an actual war, and economies are involved