What would be the point? MAD relies on everyone knowing you can take them out if they attack you. What does a space weapon offer additionally that makes the huge expense worthwhile?

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.

Space based weapons have been discussed as one of the ways to break MAD eg space based systems can see and stop a nuclear missile before it could land

Killing opponents' satellites comes to mind.

Not every target is on the ground.

Are you serious? Just because you have nuclear weapons doesn't mean all other forms of military are useless. I don't even know how to parse that argument to respond in more detail.