You're assuming Putin is a rational actor, he is decidedly not.. We're talking about the man sending hundreds of thousands of troops and hundreds of tanks, aircraft, and in some cases incredibly advanced equipment into a fucking wood chipper with zero plan B, because he thinks either the country is full of Nazis (crazy option 1), or he thinks he needs to rebuild the failed USSR (crazy option 2). An irrational actor will see his victory assured, and move on to the next USSR territory to de-nazify.
Yes, he's irrational, by degrees. Your argument is akin to saying that because someone is stupid enough to bet that the Detroit Lions would go to the Superbowl and win last year, that person is also stupid enough to cross the freeway on foot during heavy traffic.
Which again, is a slippery slope fallacy. Putin overestimated the strength of his own military, for whatever reasons, we do not know. Maybe his commanders blew sunshine up his ass about how great their military is. And he also underestimated Ukraine's own military. Possibly he had bad or biased intel.
Putin is not dumb. He's a sociopath, and a narcissist. He will continue to act in his own self-interest, but because of his narcissism and pride, he will make some bad decisions. How bad is the real question. From our perspective, we need to recognize the narcissism and pride, and not push a button that will trigger him to make the worst possible decision imaginable. If we avoid doing that, his own self-interest, particularly in his own survival, will more than likely prevail.