I think you have missed one very big part of this, the Ukrainian people would have to be ok just giving up part of their country and stop attacking to get it back. That is not going to happen as long as they have military support to repel Russian forces they will have the support to continue to push the Russians back to their own border. The Ukrainians have been pissed since 2014 the only reason they have waited till now is that they never before had the resources to force the Russians out.