What is the subset of users who are going to investigate and read an rtf file but don’t know how to approve an application via system settings (or google to do so)?

I would say quite a lot of users because even the previous simple method of right clicking wasn't that known even by power users. Lot of them just selected "allow applications from anyone" in the settings (most likely just temporarily).

In one application I also offered an alternative by using a web app in case they were not comfortable with any of the option.

Also it's presented in a .dmg file where you have two icons, the app and the "How to install". I would say that's quite inviting for investigation :)