You're incorrectly attributing the failure of paid productivity apps to the app store's structure, when in fact it is entirely unrelated. It just so turns out that the market for paid productivity apps on iOS is small, no matter what Apple does with the app store.
It sounds to me like the point is that the market could be much larger if the transaction value were clear to consumers.
In the current app store, with the methods that consumers are used to spending money on their phones, for whatever reasons, it's not.
It's like when you'll easily spend $30 on drinks on a Friday night, but suffer a shitty note-taking system for years when you could have many of your problems fixed for $5 one time just because you don't know the possible transaction exists.
People often say 'the discoverability [in the App Store] is horrible', which, I believe is inexact wording, but largely results in the outcome you raise.
It's not so much discoverability as in 'stumbling upon an arbitrary app', but more of learning about what that app does, and why it's something you want.
Once you learn about a solution to a problem you may not even realize you had, you'll be a lot more likely to make a purchase. This is why app makers jump to be featured in compilations like 'top 5 note taking apps for iOS'; other options are word-of-mouth, bundling, or being promoted somehow out-of-band.
