Lifespan of a building isn't some kind of magical threshold thing where it is perfect for 100 years and then must be replaced in year 101.
A noticeable share of those buildings built for 50 years will need to be replaced around years 25-30. Almost all of those buildings will need work before 20 years is up. Some of them will outlast their design life by a large factor.
Not to mention that not every builder is selecting the same construction techniques and design life.
Of course, pretty much only suburban areas have infrastructure/buildings built over that short an interval. Constructing 90% of a major city’s high rises in a decade would be a construction boom unlike anything the western world has seen in a lifetime.