But...
We don't have to wonder what it would be like. There's been mass use of platforms that allow "side loading" (AKA, just regular installing) and "third-party app stores" (AKA, just regular buying software) for decades.
No, it hasn't always been pretty.
Yet it just hasn't been that bad either, and the benefits have proven to be very substantial. There are incredible amounts of great software available that doesn't fit in to Apple's idea of what ought to be allowed.
And it's not like you can download with confidence from the Apple App Store either. They play a cat-and-mouse game with malware constantly and there's been plenty of collateral damage.
I think there's no question third-party app stores, or just direct side-loading would lead to a lot of new great software, without a ton more risk. I think the weakness of this argument document show that.