My point was that at the time, some people did consider plastic bags more environmentally friendly
Really? Did you meet one of those people? I certainly never did.
Also what is being argued here is not "some people" but a major motivation, which really makes zero sense IMHO. It smacks of pure revisionism and trying to blame the "other" for the very thing that the "other" warned people about, an oft used and somewhat pathetic trick used by political animals used to attempt to discredit a group they do not like. There's no point trying to take a calming middle view against such tricks - halfway to bullshit is still harmful.
how people would have known that plastic bags would be a trash nightmare
Plastic bottles already were a trash nightmare so it was kind of obvious. I spent days in the 1980s cleaning stuff off beaches.