Until very recently (2 weeks) ago everywhere I had lived was at least 150 years old - so that seems a very odd concept to me.

Flip-side of that is the timbers are all old growth redwood. If you ever poke at the stuff after 100 years as long as it didn't get wet it's fine. 50-50 chance whoever buys this place will replace the foundation. And hopefully with proper concrete. Do that and the building will last into the next century.

Yes.

Or what is your point? That we use too fine grained sand to build our cities and then let future generations deal whith grumbling skyscrapers?

The point is supposedly that providing housing for the current exploding population is more important/urgent than their great-grandchildren having to tear down and rebuild their houses in 100 years.

Well, if the foundation of the house where you live is crumbling, I'm sure you'll admit it's not good.