I interpret “society never moves backward” as a sort of tautology, to remind people that time “flows” (not literally true, but good enough for the point I’m making) in one direction and thus opinions once expressed tend to be remembered whereas opinions yet to be expressed are unknown. It is certainly possible for society to retrogress in some way, but it will always do so with knowledge of what was true in the past. Even if some information about past times is lost, people will always remember what has happened in the recent past and their current views will be shaped by that. No one knows how society will develop in the future other than vague suggestions based on theoretical demographic shifts; it is thus impossible to argue for anything based on how things might be in the future whereas it is common to argue based how things were in the past.