If Russia levels Kiev and starts moving in to the rest of the country, a) Ukraine will almost certainly not surrender, leading to prolonged insurgency, and b) I expect ground troops from neighbors, EU, and possibly NATO will come into play.
Meanwhile, all the rhetoric of Russia “saving” a brotherly nation goes flying out the window.
I'm afraid something as drastic as the annihilation of Kiev will lead to actions that are beyond the usual risk assessment levels. Countries will be compelled to act, (repeated...) threats of nukes be damned. Europe will not tolerate another Nazi Germany on its borders, period.
Put another way, a massive, discontinuous step in escalation will inevitably lead to a similar step from the other side. There is no world in which Germany and Poland go "OK then" and withdraw all aid.
Of course there is. Mass mobilization and a war economy would do the trick. Many of Putin's rivals are calling for exactly that.
Ukraine's military barely held on against 90k professional soldiers and 140k mobilised. It would not stand a single chance against 3 million soldiers and a fully militarized Russian economy. Russia hasn't even called up a tenth of its trained reserves.
Putin's rivals make Putin look soft. If they do take power, they will end the conflict quickly and definitively.