You can't make this optional because they will be "taught" how to do it by the likes of Epic and Facebook. If its available people will be exploited.
Has already happened: https://grahamcluley.com/android-security-fortnite/
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.