I agree that Apple should be pressured to allow alternative modes of payment and/or lower fees in more cases. The duopoly problem you talk about is a real and serious issue.
But on this point:
> often with the implicit suggestion that Apple "deserves" to be compensated for all of the work they're putting into hosting and curating apps
This is a huge misunderstanding about what these fees are actually for. I remember a time when major operating system updates cost a fair bit of money. The kind of free significant feature updates we see for phones these days were almost completely unheard of both in the PC and the phone market.
Apple now provides fairly significant feature updates for phones/tablets, going back many years. The App Store business model is what incentivizes this development. Doesn't matter if you don't buy a new phone.. they still make a fair bit of money on you when you use the App Store.
The App Store fees are not to cover hosting/review. It's a way to get a continuous revenue stream from the users of their hardware/software. There is only one realistic alternative to this business model: to use more advertising to extract value out of your users.. which is the path Microsoft and Google seems to be heading in. Neither is ideal. But IMO it's good that there are at least two options with different approaches to this, so we have a bit of choice.
A third alternative would be going back to paying for major OS updates. But I don't think that business model is viable anymore. People expect free updates.