On the other hand when a Dropbox user shares a file with you these days, the nudges have so gotten out of hand that it's a pain to use.

Right - you pay for the GUI and the well-balanced user experience. It's less about, strictly speaking, the storage.

Which is, in the end, true of a lot of tools where the underlying 'things' aren't particularly spectacular but rather it's the user experience that sells it

We can just all use rsync, no need for an app.

> but that there's app for mobile and desktop operating systems which deeply integrates it in the OS so it's just like a local folder that's magically synced

Which mobile OS would that be?

The big reason I stopped being excited about cloud storage is that on mobile, from what I can tell, none of the major providers care about "folder that syncs" experience. You only get an app that lets you view remote storage. The only proper "folder that syncs" I had working on my phone so far was provided via Syncthing, but maintaining that turned out to be more effort than my tiny attention span can afford these days.

Yes, notably, the File Provider extension is where the value is for me. Are there any open source options other than Seafile's SeaDrive?

https://syncthing.net/ <- like this :)

Free, opensource, works on computers and phones, can in most cases puncture nat, supports local discovery (lan, multicast).

No googles, no dropboxes, no clouds, no AI training, no "my kid likes the wrong video on youtube, now our whole family lost access to every google account we had, so we lost everything, including family photos", just sync!

(not affiliated, just really love the software)

Yeah but it seems like that can also be replicated with modest effort.

Isn't that the scenario for Nextcloud?

Yes Syncthing does those things

To me, integration with the Apple files app on iOS is critical for any Dropbox replacement (among other things).

Yep. Open source Dropbox is really Nextcloud - https://nextcloud.com