Ukraine will fall quickly if they stop getting support from the outside in terms of weapons and supply.
But regardless that’s not the question. Let’s say Russia just rolls a 20 and does far better than is rationally possible, taking Odessa and much of the east rapidly. How does that change the energy situation? Let’s say Russia captures all of Ukraine and gets its genocidal wishes. Does that change anything? Are they really gonna say “well the war is over so let’s resume gas purchases”?
I think no.