It's true that Ukrainians winning for almost a year they should be at the gates of Moscow now, no? That's what I would think if newspapers in my country (France) were to be believed.
Ukraine will never be "at the gates of Moscow" because they aren't trying to invade Russia.
This issue will be in the end again decided by power.
Anyway, I think the chance that Ukraine would continue beyond their internationally recognized borders is close to zero.
I'm not even sure what is their chance of regaining anything at all.
Who knows how this will end up, it's still very open. In WW1, Russia collapsed after about 3 years of war, in Afghanistan it took them about 10 years to retreat (and subsequently collapse).
Russia occupying Kyiv is possible, but rather unlikely. I think it's less likely than a Russian collapse.