If Putin takes Donbas, then what?

The way things are going in eastern Ukraine the Russian forces will eventually overpower the whole Donbas region and settle into a long occupation. What does the West do then? If the Russian troops can't be persuaded, militarily or diplomatically, to move out and go back to their garrisons in Russia, there would seem to be few if any options for the West to do anything other than maintain tough sanctions in the hope that Putin will be forced to relent as the Russian economy dives. It's a totally unrealistic scenario. Once Putin has grabbed the territory he was really after from the beginning, he is not going to give it up for the sake of saving the Russian economy from continuous sanctions battering. So it's very difficut to envisage how this Ukrainian war crisis is going to end without Kyiv and the West suffering a defeat at the hands of Ukraine's bullying neighbour. Ukraine will be left with a country minus its industrial eastern region, its vital port in Mariupol and thousands of valiant male and female soldiers who sacrificed their lives to try and save their nation's sovereignty. What more can Joe Biden and his fellow leaders do once Putin has achieved his territorial-grabbing ambition? I fear that when this happens, there will be a redoubling of diplomatic efforts to try and make the best of what will be a tragic conclusion to a terrible war. Even when the long-range multiple-launch rocket systems and other heavy artillery sent by the US and others are in daily use against the Russians, Moscow can pile on the agony by adding more and more weaponry to smother eastern Ukraine. With this prospect in mind what are Biden and co doing right now to stop Putin from declaring victory? Again, there are no options that offer much hope for Ukraine. I hope I'm wrong.