I don’t know if billionaires are the product of capitalism per se. Billionaires are people who have found out how to exploit the current system the best. In a socialistic society there are plenty of opportunities for corruption and exploitation of the working class. The rules are just a bit different. Billionaires definitely will defend capitalism since it’s how they’re currently winning the game, but they’ll adapt as soon as they need to as well. That or the winners will be a different group of people. Either way, the most powerful will always look for ways to consolidate even more power.