For Mutually Assured Destruction (or MAD) to work, all parties have to first understand Assured Destruction.

Then you have to make sure there are no Nihilists with power who don't care much what happens.

You also need agents who don't think war is exciting and gives their empty lives purpose.

And no decision makers who believe in magical thinking "That will never happen because..."

You also need some religious folks who believe that god's gift of life **must** go on. But also anti-religious folks who temper fatalistic talk of armageddon, judgement day, reward, martyrdom and rapture. And they have to work together.

You need a small number of players.

You also need to make sure the market for tools of brinksmanship (e.g. armaments) doesn't capture the decision making structure to sell product.

You need highly intelligent public spirited leaders of supreme discipline & confidence. And you may need to rotate them often.

You also need an enlightened wealthy class that doesn't focus on building bunkers anticipating a limited exchange, so that it focuses on stabilizing the planet on which it thrives.

You also need leaders who understand a common intellectual language with no 'false cognates'.

You may need an electorate that isn't swayed by instagram to pressure leaders into triggering catastrophic treaty obligations.

Etc

So it's not that MAD won't work in theory. The theory under which it can work is becoming increasingly implausible.

It now can't work in practice.