Interactive Brokers offers a cost plus commission structure, where your trades pay the exchange commission (or collect the rebate, if you are providing liquidity) and they just take on a ~$1 fee per trade on top. Very few "regular Joes" actually want that - what you don't realize is that exchange fees vary a LOT, each exchange easily has dozens of different rates depending on security, time of trade execution, the state of the order book at the time, and order type. Placing a market buy order when the book is sell-heavy? The exchange will actually pay you for that. Going the other way? That's a very expensive order. What if you take the order to a different exchange where the book is more balanced but the price is worse for you? It might make sense when you factor in the difference in commissions. This is why online brokerages exist, to abstract all of this complexity away.

What does democratize mean in this use case? Running direct access to exchanges isn't cheap, someone has to pay. Most "democratizing" seems to end up being opaque fees that naive customers don't realize (i.e. Robinhood)

How exactly is your magic API going to allow you to transfer money to and from the exchanges? An exchange is in the business of running a matching engine, not accepting customer deposits.

Make your own exchange.