I'm actually quite familiar with the history of app stores and getting people to pay for software on the internet. I grew up in this timeline so I have first-hand experience too.

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.