I have no data, but I'd wager that iOS developers who sell directly to end users are rare nowadays. Most of us are paid good money to build apps for bigger companies that make their money outside of the App Store. Often we don't even use our own Mac for that, so as a first step, we might use a Mac at work and Windows/Linux at home.
If we left the ecosystem, the cost of native iOS development would increase, and companies would build portable (and/or crappy) apps instead. macOS indie apps would also lose some customers, and IMHO these apps are what makes the Apple world worthwhile in the first place.
Paid apps are essentially dead on iOS and Android, thanks to both of their terrible policies. Both search algorithms for each platform place way to much weight on volume (downloads, ratings, reviews) over other attributes which they could track (like user retention.)
This means a well polished, but expensive app can be easily ousted out of a top search result spot by a rushed clone at a lower price point, even if people end up deleting the crappy one after a day or two anyways. Basically both modern app stores place a ton of value on "new" things but don't care about software built to last. So as a dev you are incentivized to abandon your old projects and just stick out new ones every few months. And on iOS it gets even worse with paid search ads which can be targeted at competitor app names.
Couple that with a 30% revenue cut, no access to your users (So you have no ability to refund them or discount future purchases), and the stores having the ability to oust you at any time and it becomes obvious that the only sustainable business is continuous crap-ware or SaaS with your main business outside the App Store. Just look at the Top Grossing chart, there isn't a single paid app till around 80 and even then the app is Minecraft.
App stores are just a digital version of any physical store.
Honestly when I go any store, I sometimes wonder how all those companies with products on the shelves, manage to sell enough goods to keep the engines running.
If we left the ecosystem, the cost of native iOS development would increase, and companies would build portable (and/or crappy) apps instead. macOS indie apps would also lose some customers, and IMHO these apps are what makes the Apple world worthwhile in the first place.