This is the assumption the left wing position is based on.

It's not "the left wing" position. It's the position that Ukraine has taken, as has most of Europe (including some conservative countries) - and nearly all the hawks in the Republican party as well, along with quite a number of conservative and/or hawkish military analysts.

But you're right that there are different assumptions about what happens in the long-term if Ukraine and Russia just keep fighting for several years without much of a change in the battlefield position. Your argument assumes that Russia can keep going indefinitely - or close enough to indefinitely that they essentially never have to leave. Our argument assumes that Russia will not be able to do that - and that like most every other large power that has been mired down in a conquest effort that's gone badly, they'll eventually decide they can't make it worth their while to stay.

While we probably won't see eye to eye on those assumptions, I think it might help if you would understand that the argument you are disagreeing with does not depend on Ukraine "pushing" Russia out. So it doesn't really matter for that argument whether they can or not. Or whether Russia is able to maintain possession of the land controlled by its army. Repeatedly pointing out that Ukraine doesn't have the resources to drive the Russian military off their land doesn't prove your position or disprove the contrary position - it's not relevant to Ukraine's war strategy because their strategy isn't based on being able to do that.

That's why it's a little frustrating that you keep repeating your mantra that Ukraine doesn't have the guns, money or dudes to push Russia out. Because their war strategy isn't based on pushing Russia out using a superior military force. It's based on making Russia pay such a high continual price to hold the lands that it can't be pushed out of that they decide it's in their best interest to leave.