MAD worked because the actors were rational, had a fixed address, and at some level had their respective peoples' best interests in mind. That is not true of fanatics, whether their belief is an economic system or a religion.
Monopoly empires like ours are fleeting, and the danger period is when it's slipping away to a traditional balance of power situation. In a hundred years we'll have a multi-polar world with fairly evenly strong power centers in the Anglosphere, the old continental European colonial sphere, and Asia. Hopefully a tripod system will be stable since if any member steps out of line they're overwhelmed by the other two. And hopefully the US will let their monopoly on power lapse without belligerence.