This seems to be mostly a construction quality issue of the time - in my corner of Eastern Europe, the 1960s buildings often are so bad to need reconstruction or replacement, but the 1900s/1910s/1920s/1930s buildings are structurally fine (with some exceptions due to total neglect) despite being twice as old. It's more of an indicator if you intentionally build mass cheap sloppy housing intended to be temporary just to allow faster urbanization ASAP (as arguably was the goal of 1960s soviet housing projects), then you're not getting proper long-term buildings - IIRC it was expected already when being built, the planned design lifetime of those buildings without major repair was just 30 years or so, and they've certainly met that bar.