Even if it's microplastic in that landfill water, it's not getting to the ocean. Worst case is it's captured with the decomposing gases and condensate, and reprocessed at a refinery or incinerated.
The plastic is much better deposited in US landfills than shipped overseas, where it may be directly spilled into the ocean, or dumped into open fields for the poor to manually sift for the most valuable material, and then there's no telling where the rest of it will end up once it rains.
Plus landfill plastic is sequestered carbon.
Even places that certify plastic recycling into a new end user product are often making worthless items that are given away so that they can claim zero waste. The material ends up as construction fill for hydraulic detention infrastructure, playground surfaces and sports fields, or even those "green" children's toys. Or cheap fleeces. Lots and lots of cheap fleeces, blankets, and snuggies.
The exact places we don't want that plastic to end up.
The hydraulic conductivity of this clay is measured in millions of years per inch. It's water tight, and it's super absorbent.
Yes, perpetual containment is what they are designed for.
Hydrocarbons can break down on their own, which is why you are supposed to change your cars oil every 3 months regardless of how many miles you drive.
It also degrades in the marine environment.
Importantly, where did you get your data from? It can't be just your imagination because that's only a tool to reinforce what you already believe.