> Even giving warnings by default about unsigned apps requesting high privileges would be fine if the implied message weren't basically "everything we haven't checked is malware". Something like "We nor any other provider you've chosen to trust have no idea who made this and we haven't checked if it contains malware. This program may steal and delete all your files. Be really sure you trust the author before running this." would be much more honest.
Well, what you ask is what's written in the very first prompt screenshotted in the blog post; it says "the developer cannot be verified", "macOS cannot verify that this app is free from malware." I don't see how this choice of words is much different from your proposal.
I don't want to go too deep into the "alternative store" discussion, it's much broader than this, but let me just say Adobe Flash. I don't think Apple will ever relinquish the strategic power to force developers to adopt APIs and track their lifecycle, and never again have to deal with the Flash scenario.
If they let the door open to "alternative stores" good luck explaining to the general public how it's not their fault if works like shite and kills hardware performance. As an example, to this day, people still rant about Apple's "proprietary music file formats" when really it's just bog standard mp4 (it's even unencrypted... you can copy it over to any industry-standard decoder and you're good to go. Good luck with WMA (if they're still around) or whatever madness Sony came up with.
The moment they would decide a major overhaul, you'd see "alternative app stores" advertising "backward compatibility", "freedom from Apple's treadmill", fragmenting user experience in an endless passing of blame about who's fault it is for the rot.