Salesmen often know what they're talking about. They could be lying, or not. Tea leaf readers, however, just make stuff up.

The combination of salesmen telling the truth about things they know and lying about things they know is, as a whole, comparable to a tea leaf reader who neither knowingly tells the truth nor knowingly lies much..

It remains true that a customer who followed your logic would lose all their valuable data

Yes, he'd be unlucky. He'd be unlucky enough to have stumbled into one of the few rare cases where being rational produces a bad result. Being told to do backups is not a typical case of listening to a salesman (or tea leaf reader). It's a highly unusual case.

Just because someone would have been better off if they had done action X, it does not follow that it would then have been rational to have done action X.