If microplastiscs are the problem we would see enormous lines in the fertility clinics around (people want to have babies but they can't for some reason). That's not the case, though. In fact, we see more and more people going to fertility clinics to freeze their eggs/sperms and not to get help in having babies.
Maybe microplastics are affecting the unplanned pregnancies. But I would argue that is a good thing. No body wants unplanned pregnancies.
I am not disputing that microplastics are bad for you or that it is affecting fertility. But the idea that they are the cause of the birth rate collapse is overblown. The effect of microplastics in the total birth rate is tiny.
People just don't want babies. Period.