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).
I imagine it's much higher, at least 5 nines, mostly because they tend to go straight from their rolls to containing garbage. They also don't blow away so easily.
However we probably should stop using them. Merely banning will just be met with confusion, since they're an integral part of the waste disposal system. But in my experience, and the experience of others in this thread, a good food waste + recycling system virtually eliminates all solid waste, and therefore the need for garbage bags. All solid waste, that is, apart from plastic packaging. Probably we should get rid of that too.