> In reality, their life span is more like 50-100 years, and sometimes less. Building codes and policies generally require buildings to survive for several decades, but deterioration can begin in as little as 10 years.

Plus or minus 25 years depending on the environment and maintenance. But I would disagree that 75 years is not so long.

Does that not depend if it’s stainless or not?

I think that's only used for structures immersed in seawater.

It's not that we can't design buildings to last longer, it's that you don't want to design a building to last two centuries when you know it's probably going to be torn down in 50 years no matter what shape it's in.

Stainless steel rebar is also used for (not only seaside) bridges etc, but at 10x the cost it's not going to be in ordinary buildings.

If a building is made harder to demolish than its neighbours it's got a better chance to survive. I plan plutonium-core concrete walls for my mausoleum to prevent future generations from interfering with it.