That's not what military experts are saying. If the nuclear fallout reached NATO countries NATO policy would require a "proportional" response, which again, military experts suggest would be a small scale nuclear strike on a Russian facility of proportional impact. The big question then is what would happen next - either both sides cool down and see where this is going, or Russia starts attacking targets within NATO at which point we're at full WW3 level. The "nobody is going to drop a nuke back" is either extremely optimistic or extremely naive.