Yeah, I mean, I'll admit it's possible that Putin would back down if there was a full NATO intervention. I am skeptical. I think that at least there'd be a fight, possibly even an invasion of the Baltic States by the Russians. And it could get much, much worse, and that's not worth risking, even for Ukraine.
The whole point of NATO is to create collective security that ideally ensures the members can avoid a war with another member of the nuclear club. And it's worked so far by avoiding direct fights, because as bad and horrible as the Ukraine war is, it would be so much worse if one side decided to make it worse.