It's at least notable that the plastics industry documented the possibility that microplastics were accumulating in the human body. But with there being such limited knowledge on what harm they do, if any, just by being there? I don't see why this would have sounded any alarms back as far as 1974?
Comparing this to cigarette manufacturers is a little disingenuous, IMO. That's an industry that encountered mounting evidence that smoking was doing harm and even killing people, yet they doubled-down, paying off researchers to try to find as much contrary evidence as possible to deny it.
It seems pretty logical that ANY substance we're exposed to can enter the human body, and many of those substances could stick around in a microscopic scale. The more important question is whether or not it matters from a health perspective. We developed some of our plastics BECAUSE they were inert materials, deemed fairly safe to use in the medical field (surgical tubing and so forth). Maybe that was a big mistake, but most of the concerns I hear voiced right now amount to wild guesses. "Look at the increase in things like obesity, or cancer! Maybe it's the microplastics!" There are MANY other possible causes in play, so jumping to the conclusion microplastics are the next cigarettes is .... unreasonable.