The solution would be a "noob mode" that disables sideloading and other security-critical features, which can be chosen when the device is first turned on and requires a factory reset to deactivate. People who still choose expert mode even though they are beginners would then only have themselves to blame.

This is just a variant of the "complicated unlocking mechanism" I was talking about. It still screws over everything not coming from the play store because the installation process for them essentially becomes a huge hassel, that even involves factory resetting their device, that most people won't want to deal with.

This should be voted higher, it quite literally is this simple.