On the plus side the plastic buried in landfills is likely to be the only lasting impression humans make on the world on a long timeline.
Like, coal is just dead trees before anything evolved to break down wood. That's why coal isn't renewable.
And there's already multiple forms of life that breaks down plastic for energy.
So those layers of mostly plastic junk will be unique. No plastic substantially older, and even if we keep making new piles, stuff will be able to start breaking it down and continue after it's buried.
Quick edit:
"Plus side" as in we're not doing more damage, not that leaving a permanent mark in the form of plastic pollution is an overall "good" thing.