I don't find it particularly interesting to argue over whether Google and/or Apple or Goople are monopolies/duopolies. I think however it is obvious that something has seriously pathologically gone wrong in the mobile software market and these two companies are behind it. I also don't think that appeals to "is that legal" are all that satisfying -- (Apple's free speech lets them silence whoever they wish, of course)…
No, its business was cannibalized by open source - OS by Linux, browser by Mozilla, native software by web applications, shrinking server installations, outflow of developers. And now they are good.
Apple closed this route with proprietary hardware and walled garden.
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Was duopoly any different on desktop?
- Apple OS X, Microsoft Windows, custom Windows builds, Linux, BSD, etc
- Apple iOS, Google Android, custom Android builds, postmarketOS, Ubuntu Touch, etc
You can't run Apple OS on generic desktop. Hardware support varies if it is not Windows - good on Linux, worse on BSD, but Plan 9? Haiku OS?
Software support is number 1 reason people stick to Windows. Linux and BSD has essentials covered - editors, browser, office. No modern browser on Plan 9, some run it on remote host, some live without it.
Microsoft (at its height) pushing OEMs, proprietary API, changing storage formats. Apple pushing proprietary hardware, API, closing ecosystem. If you play by their rules you support their decisions.
The only thing that matters is voting with your feet. There is alternative today - PinePhone and Librem 5 has open source drivers (support as long as you wish) [1], LineageOS, F-Droid, postmarketOS, Ubuntu Touch. It is not as polished but you can't have both (at least yet).