It's not a sunk cost fallacy. We cannot let Russia run rampant over other sovereign nations. Make no mistake, there is 0 legitimate reason for them to be fighting Ukraine. Letting them do so sends the message to them that we are all talk no show, and also shows China the same thing.
However, we've lucked into a great situation. Ukraine fought back fiercely; the US can just proxy war Russia through money now. No cost of human life, we aren't exactly going balls to the wall in sending equipment either. The EU gets the same benefit.
Also, I don't know what you mean by the war is costing too many Ukrainian lives. RUSSIA INVADED UKRAINE. It isn't on the US to stop donating to force the Ukrainians to roll over and accept it? In what world do you tell the citizens to just lie down and take it for their own good? That's the most asinine thing I've seen.