Would not stop what? Given the massive war crimes that Russia has committed in Ukraine, Russians still there after a Russian retreat would be wise to leave. How would a Ukrainian surrender make ANYTHING better? And more to the point of your question, how could Ukraine even START to "completely and totally destroy Russia"? This is an insane scenario...Ukraine is pushed to the limit to STOP the Russian invasion and to reclaim some invaded territory. In what scenario could Ukraine invade and hold Russian territory?