That isn’t enough to enable this business model.

It would be simple enough (yes I know I sound like a product manager” to add another rule to allow someone to download the last version they are eligible for. Apple keeps all of the previous versions of apps. They use to be available via a simple url.

There is a major update to iOS every single year and many older application simply wouldn't run.

Bad experience for users, developers and Apple.

The problem isn't the lack of possible way to do it well rather the lack of implemented way to do it well.

It doesn't work the other way though. If you have a new device, old versions will often not work.