IMO this is 100% fully reasonable as a design decision from Apple. "Regular websites" should not be able to use my device as permanent storage. 7 days is more than enough.
By having users add an app to the homescreen, it creates this great UX divide where users know that this isn't a "regular website" because it is "installed" on their phone.
I actually think this is a clever decision coming from Apple. Yes, it messes with your flow, and now you have to add a 2-sentence banner telling your users to add your app to the homescreen. No, just because it creates a small UX roadbump for your website does not mean that Apple should compromise on privacy and security.
Currently, they do not. Perhaps they will, if the outcry over this catches their attention.