I agree - Ukrainian manpower long term is of concern to me.Your opinion is that Ukraine will be the first country unable to sustain the war. My opinion is that Russia will be the one that will find it harder and harder to keep up as the war drags on. Ukraine’s backers have unlimited resources compared to Russia, and their economies are not at war, so they can continue to provide both financial and military support for as long as it takes. So from this point of view, form Ukraine’s side, the war is sustainable long term from an economic and industrial point of view. Only the manpower issue is debatable.
On Russia’s side, while the manpower pool is not going to run out, the economic and industrial capacity to sustain the war may not be sufficient in the long term. It is more likely that a country faces a total economic collapse before it runs out of people to sustain a war.
My opinion is that Ukraine’s manpower can outlast Russia’s economy and industrial capacity, and if the West continues to give incremewntal support to Ukraine, there will be a point when Russia is simply unable to hold the equilibrium at the frontlines.
Assuming the EU continues its support for democracy on its doorstep, I think that Ukraine can carry on fighting for years. Ukrainians want to retain their country's independence and culture by resisting the Russians. How many would be prepared to swap living in a free, democratic state for living under the control of a tyrannical foreign state? I think that most Ukrainians are far more motivated to defend their way of life that Russians are to end it.
No expert me but I doubt that AFU troops can retake their lost territory through soldiers retaking the ground. Russia has had the time required to set up considerable ground defences. I see the degradation of the logistics supporting the occupying Russian army as the way to beat them through forcing them to withdraw from untenable positions in areas where the population sees them as the occupying enemy. That may not be the case in Donetsk and Luhansk.
I agree that the financial burden of supporting Ukraine is not large for the EU and can therefore be maintained as long as needed. I don't see Russia being able to sustain the cost of the war for as long as needed.