We could be building small modern castles that we could hand down
While I see value in structures standing as monuments to be handed down to future generations - mostly large structures built for public use - I can't say that I've met a 100-year-old-house that hasn't had significant shortcomings. Renovations upon renovations to upgrade electrical, plumbing, HVAC have the potential to make building interiors look like a hodgepodge of bolt-on improvements. Then, one still contends with features that fall out of modern building code: that narrow staircase or the low ceiling in the basement or no windows in the bedrooms.
At a certain point, one cannot predict the housing requirements of the future and it would be more economical, and humane, to just start from a pile of dirt once again.