So, hypothetically, we're saying that Ukraine doesn't just exhaust Russian forces; it overpowers them, throwing them back to the border and perhaps beyond. In that sort of situation, Ukraine would be in a position to demand Russia recognize the 2014 borders as legitimate and recognize complete Ukrainian political self-determination.
In that sort of situation it would be reasonable for the west to permanently lift most of the sanctions as the carrot to the stick, provided Russian guarantees of energy supply. And why not? If it abides by those guarantees, it has no power over Europe. If Russia takes it out on Georgia, well, Europe will frown mightily but it's not in their neighborhood anymore. As long as Belarus continues to be run by pro-Russia regimes, anyway...
Although there really wouldn't be any way to spin the above as a win for the Russian regime, its people would most likely be better off. It would hardly be a disaster on the scale of what followed the USSR spinning apart. In terms of domestic politics, is there any possible situation where Putin gets thrown under the bus to save the regime, or is he actually the bus?