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.