> What happens if you set up OneDrive and Dropbox (and Google and Apple) to sync the same folder.
Chaos, potentially. Think about deletes... delete a file from the web UI of one service, its client deletes the file locally, but that same instant the _other_ service sees the missing file and happily restores it, the first client deletes, the second client restores, etc. etc. until you've blown gigabytes and gigabytes of network bandwidth.
In other words, ultimate control needs to be on the client side. You delete from Dropbox, and the local machine marks the file as no longer being backed up by Dropbox but does not delete it. Some UI needs to present to the user: This file is backed up in these services, but not the other one. At that point, the user has the choice of fully removing the file, restoring it to Dropbox, or leaving it alone. They may even decide they don't want it backed up in any service, but want it to remain on the filesystem.
Chaos, potentially. Think about deletes... delete a file from the web UI of one service, its client deletes the file locally, but that same instant the _other_ service sees the missing file and happily restores it, the first client deletes, the second client restores, etc. etc. until you've blown gigabytes and gigabytes of network bandwidth.