I also don't understand the alarm.
There is no hard limit on how long things will be stored. Data in localStorage might still be stored for weeks/months/years, as before.
The only limit is on how long things will be stored if the user does not interact with the site/PWA.
If you are a website, not a natively-installed app, that I haven't "used" in a first-party sense for 7 days or more, I don't think your data belongs on my device.
Storage space can be limited, and any app I haven't used in 7 days should be happy to re-fetch my data from a server or convince me to install their native app.
To act like this is some nefarious plan by Apple to get people to build native apps instead of PWAs is absurd. If a PWA was written properly in the first place, this change will have basically 0 impact on it.