I actually suspect the reason the codepath is still enabled is probably to do with third parties running in a PWA context. That said I don’t see how this is actually all that flimsy of a mechanism, it avoids needing a special case. As it is you can’t really count on browser local storage alone for long-term storage; the same is actually true for Android and iOS apps too, who lose all of their local data when they are deleted. (It is possible for at least Android apps to write data to other places like the SD card, but that is a totally different story imo.)
Honestly, if my data really matters, I don’t want it to be stored only in a single place. I can get the argument of wanting to have federated syncing, that would give the user freedom to choose where data syncs or doesn’t. But in my opinion you either care about the data or you don’t. Any data stored locally anywhere should be considered lost until proven otherwise. Like, drop your phone in a sewer, leave it in the wash accidentally, have it stolen, or even just have a different software bug obliterate your data and it’s gone. That’s the definition of fragility.
This mechanism failing is mostly theoretical, but having ones phone break is not; I would guess those of us who have been using smartphones for 10+ years have, by and large, all experienced data loss when storing data with no backup.
To relate to your statement, can you imagine if your data on Dropbox was stored on one harddrive, in one server, in one datacenter? Servers fail constantly. You can of course do whatever you want to improve reliability but without redundancy you are very much pissing in the wind.
On the note of “localStorage is temporary,” nothing in the spec defines how long localStorage persists, just that it is not bound to the session. In fact though, Safari already deletes localStorage when disk space is running low.
I am very much an advocate for folks being able to control their own data. I personally self host a lot and use a Synology NAS as my own backup for most things. But I think Safari would be wasting time to disable the counter entirely for PWAs. It doesn’t meaningfully change the likelihood that users will lose data. I think users often do want strong durability and privacy, and an API that n apps from needing to implement many remotes would be way more impactful. I’d love to tell an arbitrary notes app, “Go backup to this Synology NAS” without it needing to specifically support Synology NASes or for example, WebDAV. Put the provider on the clientside and you have a place to implement end-to-end encryption.
(Of course, Apple has iCloud backup, but I don’t think that covers your localStorage content anyways.)