I think that, with the current state of OSes like Windows and Android, there should be some minimal amount of friction to enabling installation of non-vetted apps. Maybe some switch that can't be enabled accidentally, or without understanding that there's risk involved (or at least a switch that can be disabled and password protected) for the sake of children or the elderly.
On the other hand, though, an OS should be built with enough security and sandboxing that no single application can brick your entire device without at least tapping through and giving it a ton of permissions; which means that the only remaining risk to the end user would be access to disinformation or other harmful content, or the risk of personal information exfiltration (i.e. phishing). At that point, a simple block list (or even just an allow list) maintained by a trusted guardian or third party would be sufficient to keep children or the elderly from harmful content, and whoops we've just invented the internet again.
I am once again begging for Boot2Gecko to become a thing.