Are you certain? China and Russia have both long believed that they could first strike some type of nuke in various situations and the west would not respond in kind. If Russia hit a Polish base or city with even their smallest nuke, NATO probably fractures. Massive crowds would hit the streets of European capitols chanting, "It's not worth it, peace now. Settle this!" As we've seen in so many wars, angry rah rah defiance can often spring up early in a war, but long term commitment is much more difficult when the threat becomes real at home. Former Soviet states might still maintain enough will, but I have strong doubts about Germany, Spain, Italy, etc. I also doubt that currently our own govt would respond in kind. "Ultimately we must do what saves the most lives."
Of course there are many, many ways this war could be incrementally escalated, I'm not implying that such a surprise, measured nuke strike would be likely. But it is very possible, a potential checkmate at. And your specific point was about invasion, not a nuke strike, but the threat is always there.
So much depends if Putin's endgame is just Ukraine, or a demilitarization fantasy for the rest of the former eastern bloc, or a combined Russia-China (-Iran proxy) asymmetrical cold world war three. Too much debt in Europe and way, way too much debt in the US. We're in end of WWII territory at the start of this war, with the Fed in a bad position. If the Axis of Evil actually has the ability to knock off a major part of our electric grid for an extended period of time, it could have a similar economic impact of a Great Depression. (No, I don't expect the doomsday scenario of widespread social chaos, other than some usual trouble spots we'd get through it like other wartime hardships.) If they have the ability, power outage attempts are likely if we actually try to cut off Russia's oil exports and extend SWIFT to that. If the US and Europe economies are heavily weakened, the attempt to de-dollarize energy in the BRICs may succeed. Which with our debt bomb would produce another huge economic blow to the US. A bad reset.
Then there are the critical weak points we've allowed to develop. The economic pain of supply issues of furniture/car chips/phones are nuisances and short term job losses, but China cutting off a large part of our prescription drugs would be huge. Add in some strategic terror attacks, perhaps through proxies or false flags, and we face some major political instabilities (we'll get through those, but it could further tie up us and our allies for awhile.)
Brisket's grave concern is warranted in this case. We have to play smart and sober, not the hope, wish, and denial of the past. I don't worry much about MAD, I worry about a combined Russia-China-proxies asymmetrical war. They've been wargaming and planning such for over a decade.
(/no SLXpress)