I know nobody that would uncheck the option unless they were very sure of the source, and then turn it back off again.
I've been to websites that automatically trigger the download of an apk to your phone. Without that setting it is only one or two wrong clicks and you've got all sorts of malware on your device.
It's not about competition, running a phone with that option switched off is dangerous.
Automatically triggering an apk download is just that... a download. An install (from opening said apk) triggers a fullscreen dialog of what you are installing, it's access, and whether you are sure...?
It isn't dangerous at all, you can't 'accidently' install an apk. It is a very intentional, multi-step, action.
Of course you could. You (an uninformed user, not a hacker) go to see what the notification you just got is about and tap on it. It's then only one more click to install.
Much easier and safer to have third party stuff disabled entirely.
I think there should be a way to install third party certificates for things like the amazon store, don't get me wrong, but it's a decent security feature for the uninformed masses.
But you aren't an 'uninformed user' you've gone to the app section of settings and intentionally clicked "I want to install third party apps" and then intentionally clicked "Yes, I know this isn't a great idea".
So chances are, on the spectrum you are more hacker than uninformed user.
There's a "Verify Apps" checkbox that I've seen that purportedly does this. I think the first time you install an app from outside the play store the OS will prompt you to enable this checkbox.
I've been to websites that automatically trigger the download of an apk to your phone. Without that setting it is only one or two wrong clicks and you've got all sorts of malware on your device.
It's not about competition, running a phone with that option switched off is dangerous.