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There are two separate things going on in this discussion.
You have a personal responsibility to keep your data safe. I don't think anyone is disputing that.
Independently, softwares and services which handle user-created files should never, ever delete them without a direct instruction from the user. Regardless of your backup procedures, it is completely unacceptable for this to happen. Even if I had a copy of my data in a fire-proof safe on every continent, I would be appalled if I found that a file syncing and backup service like Dropbox had deleted some of my files and was unable to restore them, because i) it's an unacceptable thing for any software to do, and ii) it completely contradicts all of their company messaging.