That only works in efficient markets, which, obviously phones are not. An oligopoly where the barrier of entry requires like a billion dollars to spin up new phone hardware, a new OS, and an app ecosystem is not a place where effective competition is going to occur.
If we require sideloading via untrusted sources, then that's just flipping the situation on its head to where, now, people who do want a phone with a single authority screening the entire software stack (from the bootrom to third party app code) don't have a phone to pick from that will suit their needs. With such a law, it becomes impossible to enter into the market at all with a product for these customers.
That only works in efficient markets, which, obviously phones are not. An oligopoly where the barrier of entry requires like a billion dollars to spin up new phone hardware, a new OS, and an app ecosystem is not a place where effective competition is going to occur.