Alan Weisman's lovely book World Without Us speculates a bit about this, basically saying that more recently built structures would be the first to collapse because they've all be engineered so close to the line. Meanwhile stuff that already been standing for 100+ years like the Brooklyn Bridge will probably still be there in another 100 years even without any maintenance just on account of how overbuilt it all had to be in an era before finite element analysis.

This is a great quote for the topic, but the quote is normally about a bridge that barely stands.

I'm chuckling at the thought of barely building something. (All in good fun, thank you.)

In my county, a company asked the Mayor if it was possible to improve some bridge because they need to carry 40t and the bridge had a sign telling it would only allow up to 32t. Their proposal was to do the construction and get tax rebates.

After two weeks, the Infrastructure department changed the sign allowing up to 45t.