I think versioning is the better option.

are you writing about losing the backUp drive?

No, losing your main version. Imagine you have a computer with syncthing and a server where it syncs to. If you chose no deletions, then it will sync all files to the server but all the stuff you deleted (draft documents, random files, photos from that time your kid held the camera button on your phone down and took 3000 photos in 30 seconds) will be deleted from your computer but still there on your server.

When you computer gets struck by lightning and everything is destroyed but the server is fine, now you have to re-sort out all your files because all the stuff you deleted is still on the server version.

Your suggestion of enabling the option to keep previous versions is probably cleaner. Personally I prefer to keep previous versions and deduplicate to save space.