It depends on how retaliation goes. If the side that's attacked uses conventional weaponry, we're all fine.
So basically if Russia starts using nuclear weapons in Ukraine, NATO could respond with a conventional invasion of Russia and be done before noon. There's scenarios where the rest of the world is fine if Russia uses nukes. But there's no scenario where Russia is fine if they use nukes.
The only leverage Russia has to prevent all of NATO from joining in militarily is nukes. Using a nuke removes that leverage.