I think iOS app distribution is a bit too narrowly drawn a market. What if Apple had 5% of the smartphone market, they'd still have 100% of the iOS app distribution market.
No, I'd say the right word here isn't monopoly, but rather duopoly. That doesn't mean that antitrust law shouldn't apply, duopoly is definitely not the same thing as a fully competitive market.
If we accept this definition of a monopoly, then the term becomes worthless. Every single device where the user can add third party software but it has to be approved by the manufacturer becomes a monopoly. Stuff like Garmin watches, "smart" fridges and other appliances, "apps" on car's dashboards (like Spotify on Tesla etc).
And why should this only apply to hardware, is it not comparable to Facebook having a monopoly on allowing games on their plattform, or the (now defunct) way of developing apps for the Spotify desktop client?
Surely, this broad definition of "monopoly" is not meaningful, and should definitely not be made illegal.
The same reasoning does indeed apply to Garmin watches, smart fridges and car dashboards since people are only going to wear one watch and own one fridge and one car.
I don't see the downside of requiring everyone in your examples that already allows third party apps to do so for free (other than passing through payment processing fees) and without delay and censorship: the costs are minor or insignificant and can be included in the purchase price of the device.
The advantage is that it leads to a fair playing field for app makers, lower prices for consumers (since the money that goes to the platform maker is all visible upfront in the device price and thus easier to compete on) and doesn't stifle niches that the platform owner doesn't like for whatever reason.
No, I'd say the right word here isn't monopoly, but rather duopoly. That doesn't mean that antitrust law shouldn't apply, duopoly is definitely not the same thing as a fully competitive market.