Or split into 2 things:

Most people have a Dropbox, Apple Storage, Google Storage or similar.

A lot of people used to happily pay for desktop software.

It is sort of a combo of those 2 things economically.

Dropbox could sweep up here by being the provider of choice for offline apps. Defining the open protocol and supporting it. adding notifications and some compute.

You then use Dropbox free for 1, 5, 10 offline apps (some may need free some paid) and soon you'll need to upgrade Storage like any iPhone user!

>A lot of people used to happily pay for desktop software.

More or less no one used to "happily" pay. Absent pirating software, they did pay often hundreds of dollars for all sort of software sight unseen (though shareware did provide try before you bought) which often came with minimal updates/upgrades unless they paid for such.

But expectations have largely changed.

Given the anemic sales of macOS apps vs online subscriptions, I would disagree. I’m sure it’s the same story on windows. Offline only makes sense when your public infrastructure is garbage. Otherwise most people will choose convenience over control.

Making people dependent on a cloud subscription isn't exactly in the spirit of spreading offline programs...

iCloud kind of does this and is the suggested way to store app data files.

It’s not immune to file conflicts across your devices though.