What happens to hot fintech startups that have benefited from a rise in consumer trading activity if regular folks lose interest in financial wagers? That’s the question facing Robinhood, Coinbase and other trading platforms that have ridden an upward cycle. Each has performed well in recent quarters: Robinhood by securing huge payment-for-order-flow revenues, while Coinbase’s trading fees have proven incredibly lucrative, something we learned when it filed to go public. The Exchange explores startups, markets and money. Read it every morning on Extra Crunch, or get The Exchange newsletter every…