Billionaires should be treated as a danger to the effective working of democracy in the same way that monopolies are treated as a danger to the effective working of capitalism. And their existence and operation should be regulated and restricted by government.
One way to do this would be to use progressive taxation to ensure that individuals do not accumulate more than a certain proportion of the GDP (or some other relevant figure). This would give a system which is capitalist in the small but communist in the large.
A #stableDemocracy would then be a voting system with an army (to prevent take-over from outside) and progressive taxation (to prevent take-over from within). There are probably a few other things that will need to be added to ensure stability against other threats.
