I blame Apple for not creating a viable system for paid upgrades; it's perfectly reasonable for a developer to expect to be paid for a major app update - even if it was largely to support a new OS - but without a subscription, the only way to do that is to launch a brand new version of your app, which loses you all of your carefully cultivated SEO / links / etc. (doing this via IAP is impractical because you can only build your app against one version of iOS at a time; it either supports the new version or it doesn't)

And I suspect Apple does this because they don't want people to have to pay money to continue using apps on a new version of iOS, or a new phone; if buying a new iPhone meant forking over $50 to upgrade your favorite apps for it, that might mean fewer people buying new phones.

So don't blame developers for this, in other words; a lot of them would be perfectly happy to charge users the occasional upgrade fee instead of a recurring subscription, but Apple doesn't want them to. (they're also very happy to have their 30% cut of all of that lovely subscription revenue)