Like Harry points out, the system has to now collapse, it is unsustainable, the value of all those billions is not really there, what can be bought with them isn’t really there, the battle between billionaires carries on, they will continue to destroy the wealth of one another, their number will progressively diminish to trillionaires etc. On the power of a people against a military, remember the military itself are also people, if the ideology to support government isn’t there, the military itself can end up leading revolutions.
Fear of that, is what can change things, no-one likes to live in fear, not even billionaires.
The "Middle classes" still have enough power and wealth to invest in things that will take the sting out of extreme wealth, like Kardashev Money, which has many of the properties of a UBI, and yet is a real source of wealth not dependent on billionaires.
Even billionaires themselves, and bankers might see the logic in this, it is surely more attractive for everyone to have assured, secure, sustainable wealth, than to be sitting near the top of a very precarious unsustainable system.