Mankind has never seen the collapse of a nuclear powered empire.

What makes us think this is possible?

It may not be possible for a nuclear powered empire to fall without the death of billions.

Mankind is in uncharted territory.

Even Russia never fully fell after the USSR collapsed. Whoever kept the nuclear weapons continued to be a first tier power.

Nothing in human history compares to the present question of how the current empires fall. And if they can even do so safely.