Oceanic plastic comes from 10 major rivers, 8 of them in Asia[1,2].
Which is to say, while landfill escape is certainly possible - when it comes to plastic, it's essentially not a problem at all compared to just the ongoing and continuous dumping of plastic into the oceans where it degrades to microplastics.
Landfills, properly managed (i.e. no liquid chemicals in them) are shaping up as the best option environmentally because they're local and while not completely frozen in time, certainly frozen enough. You dig up a landfill, you'll largely find...everything which was originally put into it.
Which is to say, while landfill escape is certainly possible - when it comes to plastic, it's essentially not a problem at all compared to just the ongoing and continuous dumping of plastic into the oceans where it degrades to microplastics.
Landfills, properly managed (i.e. no liquid chemicals in them) are shaping up as the best option environmentally because they're local and while not completely frozen in time, certainly frozen enough. You dig up a landfill, you'll largely find...everything which was originally put into it.
[1] https://pubs.acs.org/doi/10.1021/acs.est.7b02368 [2] https://www.weforum.org/agenda/2018/06/90-of-plastic-polluti...