Studies show landfills are the source for 80% of plastic waste found in the oceans, presumably coming from badly managed landfills where garbage washed out by rain, or illegal dumping.
There does not seem to be definitive data on the source of plastic pollution in the oceans, so I would really like to see the sources for "80% ocean plastic is from the landfills".
If we know how they end up there it will help understand how well solutions will work obviously. Are people flushing them down the toilet? Does the wind catch the bag before anyone can react? Do people throw them on the ground because their pockets are full and there is no trash bin for miles?
The minutiae doesn't matter. Every bag that doesn't end up in a landfill or incinerated, will end up causing trouble in the environment. You know the disposal rate won't be 100%. Why worry about the specifics?
Because every environmental policy should be based on cost-benefit analysis. Otherwise we could just ban all plastic, all fossil fuels and much more.
It seems to me that you, in this case, intuitively think that cost-benefit analysis sways entirely one way even though you don't know the specifics. But the bar for banning stuff should be higher than that.
It's not just about this particular policy. Every time a restrictive policy is introduced by hand-wavy feel-good arguments (even with best of intentions) it becomes more acceptable to introduce other future policies in a similar way. And if it doesn't actually produce results it's actively harmful - we feel like we're doing something but we're not. Maybe we should just forget about plastic bags and invest in putting more garbage cans around the city or ramp-up inspections on landfill management (thus potentially solving the problem with plastic bags, straws, bottles, utensils etc. with one stroke). How could we even tell? By knowing the specifics.
And we have not even started to discuss alternative, less heavy-handed measures, like levying a tax on each plastic bag or compulsory minimum prices for them at the store (seems to work great, according to some commenters. It was introduced where I live some time ago but I didn't notice any change because there wasn't much litter even before the change).
To play devil's advocate a bit: If most of the plastic bags in NYC were coming from New Jersey, that wouldn't be a reason to allow them in New York. It would just emphasize that they should be restricted in as many jurisdictions as possible.
I don't think that particular reasoning works - perhaps New Yorkers (hypothetically) behave very nicely and properly dispose of plastic bags, so there is no need for ban.