>Sounds good. Where is such program? This is surely not a very novel idea, but where is it?

Every office in the nineties. Windows 95 + Office on the desktop and a Windows NT Server sharing the files. I'm not saying we should try and wind the clock back but it was a solved problem.

It's probably less that they suck, and more that they aren't designed for the workload the developers want. Files are a good idea when you have exclusive write access to them at any given moment; less so, when you want to support concurrent access. Real-time collaborative creation happens at a finer level of granularity - people are manipulating individual aspects of documents, objects in the application's internal model. This doesn't work well when your atomic unit of synchronization is entire database.

Not that I disagree with your overall point, but there are multiple products in that space. It's clearly not a deal-breaker for most; but if anyone's lamenting the lack of a GoogleDocsFS, they can get one:

https://mountainduck.io

https://cloudmounter.net

https://www.expandrive.com/desktop/

https://github.com/pranavmk98/gDocsFS

I mount a cloud drive containing about 3 TB data on my laptop using rclone and it works great. And the cloud provider does not even have a native Linux client. I am so happy with rclone and will totally recommend it.

https://github.com/rclone/rclone

sshfs sucks, sure, but there are other file system protocols. Plan 9's 9fs is pretty cool.