I too have a 200 year old house and the sagging mentioned in the article seems inevitable in wood.
However plastic is a great material for dark, rodent free places.
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))
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.