If there was a use for collected plastic, a lot less of it would "get away", currently most of collected plastic ends up in landfills anyway. Burning jet fuel obtained from plastic would not decrease CO2 level, but it would not increase it either, as there is lots of other jet fuel to burn.
If it were profitable to turn plastic into jet fuel, that would imply that plastic-derived jet fuel is cheaper. If the price of jet fuel goes down, more gets burned.
By that logic you can argue that banning plastic bags will decrease the use of oil, making it cheaper for those who want to burn it. Or that buying tesla makes oil cheaper and in some other place more of it gets burned. In all of these cases the effect is tiny.
It's a question of supply and demand. If your final goal is to burn less oil, then you want to 1) reduce the supply and 2) reduce the demand. Buying Tesla reduces demand: good. Turning plastic to oil increases supply: bad.
Your plastic bag example is interesting because I think it's accurate - having plastic compete with combustion as a use for crude oil is good from an emissions standpoint, because when the wells run dry, less of it will have been burned.
(Plastic bags are indeed a bit of a silly example because there's so little plastic in them - we ban them because they cause disproportionate environmental harm)
