MAD is not military doctrine and never has been. It's a philosophical concept that arose in the mid 1960s and gained column-inches in the press because it had a cool and ironic name. Even McNamara rejected it.
USA, China and Russia all plan to fight and win wars, not go out in a blaze with the other guy. And that includes 'nuclear warfighting' which involves the ability to command and control nuclear exchanges and come out the other side victorious.
I would argue that despite not being military doctrine, it has been effective. Nuclear deterence is demonstrably real. It is a form of Nash equilibrium.
I am not sure there is any limited nuclear exchange that is not so risky as to be worthless unless you just want to die.
Space based nuclear weapons do not improve your survivability. Space based nuclear missile interception might - if you could pretty much ensure you could take out the majority of your opponents missiles (star wars).
The problem there is it destabilises the Nash equilibrium of MAD and makes it more likely someone will initiate a first strike before your defensive capability is complete...
USA, China and Russia all plan to fight and win wars, not go out in a blaze with the other guy. And that includes 'nuclear warfighting' which involves the ability to command and control nuclear exchanges and come out the other side victorious.