This is not entirely accurate. It's only script-writable storage. HTTP cookies are not removed.
I'm not defending Safari's policy, by the way, just describing. I think it sucks, and a conspiracy theorist might note how it favors native apps over web apps.
Needing to re-override the incorrectly detected mobile/not upon return after a full week away will be massively less annoying than needing to do it every new page request, so that shouldn't be problematical.
That Safari does this is useful information that I may need to warn users of one of my projects about, as it means intentionally offline data has a much shorter expiry date than on other platforms.
Everything you've not visited in the last seven days, yes.
Things you touch regularly should be fine.
And apparently it only affects mass local storage, not cookies which are most often used for season management (so you might stay logged in but the app need to reset data previously called in local storage).
I'm not defending Safari's policy, by the way, just describing. I think it sucks, and a conspiracy theorist might note how it favors native apps over web apps.