The situation is complicated, and nobody wants to have an honest conversation about it.
The reason why the switch to heavier bags is important is mainly to stop them from blowing away in the wind when people litter, where they end up in the water system. I don't think anyone really has any serious concerns about the density of plastic that ends up in our landfills, ideally, never to be seen again. The idea that plastic is bad, without concern for whether or not it ends up in a landfill, I think is misplaced concern. There are some decomposition GHG concerns, but again, those are insignificant compared to something as common as just driving ever day. Here we must remember that recycling plastic is generally a bad idea altogether if we care about GHGs.
The only people that seem to be pushing back against the bans are people with sudden and politically surprising (fake) environmental concerns because they are annoyed that they have to pay 10¢-25¢ for a bag (and absurdly trivial amount), and having to ask for one, instead of getting them for free without asking. This also has the effect of making paper bags competitive with plastic.
The entire debate is between most people on this issue seems to be people who either don't understand what it's about or don't actually care. Virtue signalling on the left or fake concern on the right.
Most people are happily ignorant of anything that doesn’t reinforce their priors.
You still have no proof at all on the table for your speculation about the plastic items on the beaches.
You are assuming that people are throwing away more reusable plastic bags. Are they?
Where I live (Sweden) the extra plastic bag fee introduced made people also buy more single use plastic bags in bulk which were cheaper and flimsier. If they are making up a larger part of the "items" counted, and not the reusable bags, then the win is even greater than a 25-47% decrease.
What reason do you have to believe people are throwing away the heavier, reusable bags at that rate? Do other bags not exist anywhere?