At a certain point (probably to a large extent already) Ukraine will simply run out of manpower. Demographically it was in a very poor state to begin with to such a degree they had to keep the MINIMUM age of conscription at 27 and lowered it to 25 a few months ago.

There were only ~2.6 million men aged 15 to 30 and another 3 million in their 30s back in 2022. Around 0.6-0.8 million Ukrainian men have left the country for the EU (18-60, but I assume it's highly skewed towards lower ages).

A significant proportion (probably the majority) of those that remain in the country are not particularly motivated, capable or otherwise keen about going to the frontline. It's hard to tell but looking at estimates > 150k have died or been severely wounded and presumably a several times more suffered lighter injuries.

This isn't WW1/2. Poorly trained and/or highly unmotivated men are not very combat effective and mobilizing such a large proportion of population as back then is not feasible (especially considering that men in their 30s and 40s have been doing most of the fighting). So how long do you think Ukraine can hold out if we extrapolate the casualties rates from the last 12-24 months or so? By the time the West fully ramps up military production it might be too late.