What is behind your thinking that plastic won’t last? Plastic is estimated to take 1000 years to decompose in landfills. Obviously it did not exist millennia ago, but I wouldn’t write it off as not future proof.
I too have a 200 year old house and the sagging mentioned in the article seems inevitable in wood.
Plastics break down in sunlight, becoming brittle and easily broken. They're also basically impossible to repair if they do break. They can also been eaten by rodents.
However plastic is a great material for dark, rodent free places.
Plastic breaks down in tiny pieces and is almost impossible to repair when broken. It's also very difficult to reuse or recycle when you want to. If your house is still standing after a thousand years, it's also because its habitants were able to remodel it without destroying its materials.
As a matter of fact, the amazing carpentry I was talking about previously is made from "fresh" oak, but a significant part of it was also taken from a previous building (church, farm, stable or monastery). It's doing just fine, apart from a few out-of-place mortises. I tried reusing plastic pipes, once, but that wasn't a great idea. It's fine for a few DIY, though.
Stones, bricks, wood are great mostly because they can be reused, but also because they will continue to look great afterwards.
(Note: You might argue that "your" wood is not easy to reuse as well. This happens because we put nails and screws everywhere and we prefer less-dense wood over heavier ones (pines vs oak))
plastic becomes very brittle very quickly (years), and falls apart into tiny bits. it's these tiny bits that (we expect) take forever to decompose.
even if we understood plastic to retain its original qualities over time, it is impossible to speculate about a thousand-year lifetime. it's unusual to come across any plastic a hundred years old, intact or not.
I too have a 200 year old house and the sagging mentioned in the article seems inevitable in wood.