We would need to find a way to give money to people so they can keep participating in the economy even though everything is cheap. If not UBI, we would need to find ways for the majority to do something that is not automated, and give them some coins in exchange.
For millennia the currency has been energy (human labor, then machines) and intelligence (human intelligence, then artificial intelligence). If energy and intelligence price goes down, and the amount of energy and intelligence increases, then what is left for humans to claim some reward/coins ?