Computers overflowing with naively self-installed adware/malware is a recurring and persistent problem with some relatives of mine. Nothing you can say stops them from inadvertently doing it again, they simply lack the sophistication to understand, and then complaining that their computer is slow or doesn't work anymore. Putting these people on iOS is a godsend, these kinds of issues don't happen there.
So users can lockdown the ability to install stuff if they want without overcoming various hurdles (maybe allow users to add customised message so if a user tries to do it a message will pop-up saying "Your Son/Daughter/whoever has said you should never disable this! Call them before doing this if someone has asked you to!").
I feel like there is so many options/info hidden from consumers about their devices that really shouldn't be. And preventing it or hiding it only really serves the companies themselves, not the end user.
Has already happened: https://grahamcluley.com/android-security-fortnite/
Fortnite is an immensely popular game people search for to buy, whereas malware etc are almost by definition not something you think "gee whiz, might go buy that".
For the type of user being discussed it seems very simple to just say/have a setting of "only allow downloads from official app store" combined with the above. Although tbh this may all be rendered moot by AI LLM security style tools that can actively monitor and prevent users from doing stuff like this.