If Putin takes Ukraine, it will become a buffer zone to protect Russia. His next target would probably be Finland. He wants to create barriers to protect Moscow and St Peterburg.
The Donbass is important as it makes it much easier to get to Kyiv in the future.
Putin might settle for a peace deal that is short term if he is given the Donbas.
Then he will attack more of Europe. His plan is clear - to bring back Eastern Europe into Russia. Perhaps even the West of Europe. He would have died from old age long before then.
If it becomes apparent that Putin wins the Donbass, Europe needs to join the war.
If we lose Ukraine, that is 40 million people that Putin will use as cannon fodder with experience in modern-day war conditions