> Plastic pollution is caused by bad management of it (and accidental loss) that spreads it into nature.

The idea that nature is over there, not here and that the landfill site is somehow ok to ruin has strong vibes of the (fantastic) comedy segment where the oil spill happens outside the environment.

https://m.youtube.com/watch?v=3m5qxZm_JqM

That is a humorous distraction. We need to put waste plastic into a billion bins, the billion bins empty into fewer larger bins and landfills are the final bins. The landfill area required for this eventuality is tiny relative to the area of environment which we consume for many other things - even car parks alone consume far more of Earths environment than landfills. Airports too, and Mines, Roads... all themselves dwarfed by agriculture. Properly situated 'big bins' have far less impact on surrounding places than those things.

There is quite a bit of difference between an spill and some contained contamination of a very small volume.

My home directly overlooks the River Thames. From my perspective, "contained" isn't the right word.

Okay...? Contained would mean not going in rivers, obviously.

Why is it not the right word?

"Contained" implies that the problem is under control. It isn't.

That's what it would be if the bad management was removed. The problem being fully under control is theoretical, so pointing out the real world isn't under control is not a counterargument.