The issue is not even who watches the watchmen, it is who is capable of holding the watchmen to account. If you build a system on economic incentives alone, the only accountability mechanism is the market. If the incentives align to act maliciously, you can expect the expected. Nation state regulators are paid very little relative to the financiers they regulate but if they act corruptly and get caught, they go to jail. Blockchains will never have this string to their bow and it is consequently extremely difficult to envisage them replacing the current financial system.