I would like a world where buying something means you get final say over how it operates even if you might do something dangerous/harmful/illegal.

I would like a world where I have the final say over whether I should have a final say.

One way to achieve this is to only allow sideloading in "developer mode", which could only be activated from the setup / onboarding screen. That way, power users who know they'll want to sideload could still sideload. The rest could enjoy the benefits of an ecosystem where somebody more competent than their 80-year-old nontechnical self can worry about cybersecurity.

Another way to do this would be to enforce a 48-hour cooldown on enabling sideloading, perhaps waived if enabled within 48 hrs of device setup. This would be enough time for most people to literally "cool off" and realize they're being scammed, while not much of an obstacle for power users.