There is a product out there called QBox that allows you to lock and sync the file with multiple locations. The downside is that remote users can only access the file when the local users have closed out of QuickBooks. Kind of a neat product but again not ideal when you need multiuser access all the time. The best solution in my opinion is an RDS server with the QuickBooks data file stored on the RDS for best performance.
As the QuickBooks desktop products seem to be coming closer and closer to an end, I agree that the online version is likely the best way to go. Part of the problem is that users have been using the desktop version for so long that the online version becomes very foreign to them and most want to return back to the desktop version.
Yeah, that's not multiuser but rather "rapidly changing and coordinated single user". Could be a useful solution for some, though. You can do this with most products. NextCloud for example.
From the use case I've seen that is correct on being a single user product. I did watch the video on the link I posted and it does look like you can still do multiuser as long as the users are all on the same network. The remote user can only access once all the local users are out of the file and it has done it's sync. The file locking is automatic as well which is nice. It basically prevents the outside user from accidently overwriting your changes.
Sure, you can always bypass the sync and do local multiuser, but the humans have to manage that.