This is the paradox. Coinbase does not gain anything if bitcoin gets better. Their business is "we make bitcoin use easier". If both users are Coinbase customer

This is the paradox. Coinbase does not gain anything if bitcoin gets better. Their business is "we make bitcoin use easier". If both users are Coinbase customers, their transaction is done outside of blockchain. Their goal is to have more users and wonky or slow transactions outside of their servers helps them.