Why? Isn’t Crimea an example? There is indeed insurgency in Donbas, except it fights against the Ukrainian Army.
I understand that the scenario is hypothetical, but if you are a Ukrainian and there is a Russian invasion, what are you going to fight for? Kiev oligarchs or Hunter’s business interests?
First, in 2014 Russia was already in Crimea, with military bases, per agreement with Ukraine. Second, in the Donbas the population is heavily ethnic Russian (compare this to the Sudeten Germans in 1938), and would welcome returning to Mother Russia. Third, Ukrainians west of the Dnieper want to remain independent. What Putin is actually encouraging is a hot Ukrainian civil war.