I think what you might not be seeing is the entirety of the life cycle and consequences of each action. I am
not making the argument that plastic bags are better/worse/neutral. I don't know enough to make that argument at the moment, even if I wanted to. What I am saying is that very few actions come without consequences. And, more often than not, those consequences are not immediately obvious.
Consumers, as a generalization, are pretty dumb. They'll get behind causes because they are told to do so by politicians, news organizations and celebrities. They --the generalized masses-- never use a process that involves critical thinking and informed analysis to make decisions. Perhaps more important than that, they never stop months or years later to evaluate the real outcome of the decisions they made. That's just how things work.
Banning plastic bags did not eliminate the need to carry things. That means a range of solutions surfaced. That ranged from thicker bags to paper bags, both of which were sold as better solutions. And yet, we don't really know. For example: How much more plastic is used to make the thicker bags? How does their entire life cycle rate in terms of energy, resources, pollutants, CO2 and, yes, disposal? Same question about paper. In fact, with paper we can even add questions about deforestation. We need trees to capture CO2. Are we cutting down more vegetation because we banned plastic bags and straws? Are we generating more waste and CO2 because of this and, at the same time, capturing less CO2 because we need more trees?
Again, I am not taking either side. I have written this comment merely to point out that the real effects of any decision are never --ever-- as simple as what we might each see individually. The fact that you don't see plastic waste washing-up on beaches might have no correlation whatsoever to a measurable improvement in waste generation at all.