Forcing everyone to use a decentralised system where anything goes "wild west" style is probably a terrible idea.
I don't think that is what is being argued.
What is being argued is the right to use a decentralised system over a centralised walled-garden, if you so wish.
I'd rather my mum used a walled garden where the apps are (probably) safer and much less scammy.
F-droid is a great app repository, no problem with them whatsoever. I am highlighting the fact that a purist argument for a technological change that does not extensively invest into understanding the negative impacts on consumers is bogus. How many iPhone users really need an alternative store? Versus how many iPhone users want to have safeties around installing apps critical to their well-being?
To your point: maybe a hard to enable setting for allowing sideloading would satisfy both the safety and the flexibility concerns. But at the end of the day, if I ever need a hackable device I will just get an Android or jailbrake my iPhone. I explicitly separate my own needs from what I perceive as a very dangerous change for 99.99% of iPhone users.
We have recently implemented some rudimentary controls where you can use Device Admin mode to lock F-Droid to a given set of repositories. That strictly enforces the walled garden, but doesn't require a single monopolist have all the power.