Let's consider what would happen if Apple allowed a "purchase" link, as you suggest.
I would expect that *vast majority* of apps would, very quickly, do this. It's a fairly simple thing to launch Safari from an app. And it's not that much harder to jump back into the app after successful purchase. So, just saying "Apple should just allow developers to put in a 'purchase' link" is kind of silly. Apple would be cutting their app-store revenues down to a fraction of what they are now. There's no way they're going to intentionally throw away all that money.
I agree, and as others have tried to point out, it's foolish to assume that the App Store would continue to operate the same way if a ruling went against Apple. The sole purpose of the App Store is to make money for Apple. If Apple were forced to forgo the subscription revenue, they'd find another way to make money (maybe charge per app download) or shut the store down. They aren't a charity.