There's no law against a more democratic way to implement the broker either but it requires interesting methods of coordination and/or decision making that doesn't seem to exist yet?
Seems like it wouldn't be much of a stretch to compare that statement to not starting a business because the economy is unfair. People indeed don't start businesses when the bureaucratic or tax overhead outweighs the financial benefit, but nobody loses sleep over an individual's hypothetical missed opportunity to learn a new skill but them. Doesn't matter to the platform owners unless it also stops being profitable, so it's their job to maintain the profitability for their ecosystem despite whatever barriers they put up.
It's not enough to not have a law against it, we need to have and enforce laws requiring it.
Developers are businesses and the economics need to work. For that, safety and security is much more important than openness.
Meanwhile, you're not looking at those who left, or those who decided to never enter a broken market dominated by players convicted of monopolistic practices.
This seems much more intuitive than a hypothesis where somehow people would prefer to enter a closed market over a fair and open market with no barriers to entry.
Remember, monopolists succeed because they are distorting the market, not because they are in fact the most efficient competitor.
Before the App Store, the picture was mostly a disaster of security, reliability and quality. There was no trust and so people didn't bother parting with their credit card information to buy software...especially not on their phone.
Apple's App Store model dramatically grew the pie because it was one of the few platforms that people were willing to actually transact confidently on and trusted. This is why millions of developers flocked to the platform. This is also why Apple has traditionally maintained an iron grip on it; it was beneficial for everyone involved.
Over time, they are being proven right as more open platforms realize that openness at the expense of trust doesn't work for the masses.