> 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.
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.
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.
I suspect that is the parent commenter's point: If you can't externalize the problem, you are forced to deal with it. If you have to deal with it, chances are you won't just dump it in a landfill.
Plastic would just wind up in domestic landfills. It's not like countries are running out of landfill space.