By that logic, the world should have just given Hitler everything he wanted too, right, because Germany leaving would never happen. The Soviet Union left Afghanistan when the costs got too high. So did the US. Under what basis do you aver that Putin leaving Ukraine can never happen?

There is a legitimate debate as to what cost is worth paying, if any (as well as what cost comes from not doing anything - ask the Baltics), but to present the situation as fait accompli?