I don't buy this argument. Developers will work on platforms that have users with money to spend, even if the development hardware is poor value. In this sense, Apple has us hostage!
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.
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.
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.
Amazon.com wasn't a Windows desktop app even though most customers would have used Windows to buy from Amazon. You could say that shifts like the Web are inevitable and nothing Apple could do will change that. That's probably right, but the way in which it happens and the role old incumbants play in the new world does depend on whether or not they are hated and actively avoided by the new elites.
I believe that Apple is making the upcoming decline steeper and deeper right now.
Now, as they make the Mac worse and worse to have, who knows, maybe people will finally be motivated enough to build an Xcode that runs on Windows or Unix. But you'd still have to have at least one Mac to do your publishing.
But where is the excitement? Where are the growth opportunities? Where's the space for experimentation? Where are things moving?
Once all the wealthy people on this planet have their iPhones and their six favorite apps in their six home screen rows, all extremely vetted by Apple, the market is saturated and stagnant.
You can't skate where the puck is going to be because the puck has come to a complete stop and all players are sitting on top of it.
I'm a Mac, iPhone and iPad user. But would I ever build a _new_ company on that platform. No. Definitely not.
Maybe iOS isn't the new platform, but it's certainly a good place to start. Still miles ahead of the next few options.
I buy all of my Apple hardware used from third parties. I don't buy any apps, music or books from Apple. I only use free apps.
The only time Apple gets a cent from me is when I pay for my developer license.
When I show my customers what a giant PITA it is to work with Apple and iOS, many of them start planning a switch to Android or Windows.
I've been single-handedly responsible for moving entire companies away from iOS in one way or another.
Would that turn into a downward spiral? Don't know.