NATO is ensuring it’s just about sustainable.

Yes, by pushing to reduce the drafting age, which the Ukrainian don't want to, because someone needs to raise families and Ukraine has a way lower population, while Putin brings in north koreans already. And can also escalate by general mobilisation at some point if cornered. So I don't see the outcome as clear.

It's not sustainable for -either- side.

Both sides are aching (very badly) for this thing to be over, or at least taken off the stove.

One can quibble further as to the details -- which are a matter of metrics, wildcards, politics. And (as recent events have shown) there are still many cards to be played.

But that's the fundamental equation we need to keep in mind.