The point isn't necessarily to be able to wage a conventional war against Russia or China. Other countries, like Iran or Syria, potentially have access to the same weapons systems.
[1]: "Spartans in Darkness: American SIGINT and the Indochina War, 1945-1975", pp. 261-263. https://www.nsa.gov/Portals/70/documents/news-features/decla...
Suppose Russia invades Poland. With a conventional deterrent, NATO can deploy ground and air forces to a defensive front and hold Russia to a stalemate. Eventually it would escalate to a nuclear exchange, but only through a stepwise process. Most importantly, this is why Russia doesn't invade Poland.
Without a conventional deterrent, Russian tanks could reach Brussels and the only countermeasure would be a nuclear first strike against Russia. That wouldn't happen. Which means there's no reason for Russia not to invade every non-nuclear-armed country it wants to. Which means every single country in the vicinity of Russia is going to want their own nuclear deterrent, because that will be the only effective defense.
(Also, missile defense is getting good enough that MAD might not survive the 21st century, and then we'll feel silly for not having any other military options.)
https://en.wikipedia.org/wiki/Credible_minimum_deterrence
https://en.wikipedia.org/wiki/Minimal_deterrence
Basically, it's not "I can glass your whole country" but more "I can nuke a few of your main cities, you will survive but it will hurt".
Mostly, only the USA and Russia can follow MAD, the other nuclear powers with a few hundreds warheads, follow Minimal Deterrence.
In what fantasy world does anyone actually posit it not devolving quickly into nuclear war with these nations?
We've had MAD for half a century and yet today so many people seem to spruik this idea that conventional direct warfare is possible between nuclear armed states. It's simply not going to happen.