I think 50 years is a very pessimistic (and journalistic) value. There are whole towns in eastern europe build with reinforced concrete and they are still ok.
They often aren’t OK. In many of those whole towns in Eastern Europe, the municipality is gradually pushing to move people out of the socialist-era concrete buildings and into newer ones. Their design means that, even if the structure itself does remain physically sound, they often struggle with ventilation, so mold forms.
That’s not universally true. In my home city the socialist-era blocks tend to be the ones without mould, when compared to some of the under-regulated new builds. If anything they’re sought after.
True, but unrelated to the point being made.
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.