Ah, but that theory of what is realpolitik is turned on it's head by clever people. Putin decries his war as a response to Western and in particular US influence. It seems to me this necessitates a re-evaluation of what is realpolitik in such a situation. Thus, who is driving? Ukraine is driving.
Public narrative isn't policy. Realpolitik isn't one view but principles of how to view things, for perusing ones national interest at the expense of everything else, roughly put it is a Trump like transactional approach that looks at your bottom line vs more idealistic Biden approach that alienates the biggest oil supplier in the world due to its human rights abuses, you can see this in Russia approach as it works closely with Turkey despite Turkey supplying weapons to Ukraine against them.
Regardless, the point here is that war is about power (who has it, who doesn't, and who can effectively use it e.g. Putin's fuck up was due to over estimating his power believing the west can't effectively use their due to political and economical realities) and that without outside support Ukraine military will collapse, so while Ukraine is in the driving sit the reality is that it has to maintain a dialogue to avoid spinning out and crashing.
Western support is Ukraine's full war mode, without it its military demise is inevitable due to simple economic starting with ammunition, and all it could ever hope for would be guerilla resistance with Syria/Chechnya like devastation with Russia easily chocking every access point into the country.
You seem to underestimate how hard it is to take a single large fortified city, even when you have total air supremacy and are willing to reduce the city to rubble and the other side has only light infantry. And Ukraine is a quite urbanized country
I said that without western support Ukraine
military demise is inventible (emphasize on military). Otherwise, you seem to underestimate how Ukraine war effort dependent on outside support, for example how long do you think that Ukraine AA can hope to last without expansive ammo, NATO early warning capabilities, fuel communication, etc.
Edit:
Wow, somebody never heard of Vietnam, nor Afghanistan(Russian invasion and American invasion, for the uninformed)...
Great examples of wars pursued without outside support.. /s