There's a very practical reason for the "do not negotiate with terrorists" mantra. Not because it's good or noble, but because negotiating simply kills much more people in the long run, probably orders of magnitude more.

The west did exactly what you said the last two times Russia took a bite out of Ukraine, and now we're in this mess, with millions involved and nuclear war on the table. I fear to think where we'd be with another round or two of passing the buck.