You have a point but it's also kinda flawed. The marginal cost to pick up a thick plastic bag is the same as that of a thin one - perhaps even a bit less as it

You have a point but it's also kinda flawed. The marginal cost to pick up a thick plastic bag is the same as that of a thin one - perhaps even a bit less as it is less likely to tear and disintegrate, and more likely to either be reusable or return some of its stored energy if burnt. So if there are only half as many bags to pick up, the cost of mitigation goes down proportionally.