If by some miracle they conquered the whole of Ukraine tomorrow, they'd still have an insurgency to deal with.
They've stoked the flames of Ukrainian nationalism. Once you've killed enough babies, there's an almost unending supply of people who don't care about victory, all they want is revenge. These are people who look like them, often speak the same language, and often also hold Russian passports.
It's already a huge issue in the territory they do hold and in Russia itself. Terrorism and insurgency will be a problem for them for a hundred years.
The territory they've gained will never make them money. It'll be like Northern Ireland x10.