People should want something they don’t want.
Because…
People who want to make that thing would like a job making it.
But it’s backwards, you can’t say that people “should” be a certain way you have to look at how they actually are.
Being passionate about building something does not convey a right to a job doing that. It makes for a fine hobby - but outside of that build something people want.
I, however, would like to pay people to make this thing that may just happen to have a passionate origin. I also think many people want some polished version of this thing to exist, despite not wanting to fund its manufacture and maintenance. I think the inconvenience to the user in the interim is worth it, but many/most don't agree with me and that is one reason it doesn't exist.
Making legislation to force third-party marketplaces to exist on a "platform" is more in line with your concern, to be honest. It's forcing an entity to do something it doesn't want to do. If consumers cared about this, they would move to a different platform or gasp create a competitor that does what they want. I already know of some people that have moved from Apple to Android simply because side-loading is possible.
Also, I take issue with the notion that in order for something to exist today, it has to have "broad mainstream appeal". A marketplace by its very nature benefits from diversity of marketed goods, but just like any other product, it shouldn't need to completely displace market leaders to exist. The rise of megalythic companies, VC being promised eventual Google-scale growth of their bets and people's general squeamishness at using anything at all unpolished or inconvenient has created this world where everything needs to be huge in order to be considered successful in any meaningful way.
F-droid is a thing and I don't think anyone would argue it
A. Doesn't exist.
B. Is a real competitor to the Google Play Store.
C. Doesn't invovle people in the market, both developers and users, who are happy that it exists and wish to further it's growth.
I can say, I think a lot of the problems we currently have and ones we are trying to spotfix with legislation could be better solved via a more conscious consumption. "Should" is a subjective term based on a cost/benefit analysis and while I think people should act differently, I don't think they should be barred from acting as they have today nor are they morally corrupt for doing so.
Everyone buckets and weights the pros and cons of everything differently.