Russia's military force currently relies on men willing to die for money. That could change. But Putin seems reluctant to force the general population to die in Ukraine.
Classic economic theory suggests that the amount you need offer to people willing to die goes up over time.
For Ukraine the main thing is to get to the point that Russia doesn't attack any more. There is no need for Ukraine to concur any part of Russia. Even getting the currently occupied land back is mostly optional.
> Even getting the currently occupied land back is mostly optional.
That's only true in the short term.
If Russia gets out of the war with Ukraine with territory gains, that only serves as incentive to start up again after Russia can regroup. After all, Putin's stated long-term goal is to take the entire country (among others) and restore the USSR.
Of course, taking back the occupied land is also easier said than done, as it would severely weaken Putin domestically to have expended all these resources and lives for nothing. There's no way he can allow that.
Both countries are in a catch-22.
This war has already changed. Near-stalemate on the front lines, exchange of strikes on civilian infrastructure (Ukraine made to Belgorod what Russia made to Kiev). It‘s a nuclear war without nukes, aiming at strategic defeat without advancing armies. And Russia definitely has more resources for it.