But what if they disappeared entirely and we built something new in its place? What if the entire block was completely transformed at some point?
I remember a celebration in the late 1980s for a retiring British architect/art historian, who reminisced about being an architecture student during the Blitz in London. While he and his pals sheltered underground during the bombing, they shared a (slightly shameful) excitement about the prospect of rebuilding London with "something new" after the war was over.
In contrast to some European cities, where the goal was to recreate as much of the pre-war architecture as possible, postwar London was, as I understand it, rebuilt with lots of new architecture. My impression is that most people think a lot of it was pretty terrible.
I can empathize with modern attempts at buildings looking ugly and soulless. But I feel that’s part of the point of transience - the next generation can rebuild it in their style. It’s also the problem with how every city becomes overpacked and unlivable when things change, as nothing is future proof.
We shouldn’t continually impose our choices (architectural, zoning, location) on all subsequent generations.
As for building something new... What exactly would need to be new to justify the construction? Density in cities only continues to grow, but that density is solved by verticality rather than replacement. You build up and you build onto rather than build out and build over. If it's to replace architectural styles, that is something that has to be decided on an individual example, as there's things like a city's aesthetic identity, historical significance, and architectural cohesion to consider. Erasing Lancashire's infamous council houses would be decried for decades despite their homogeneity and and connections with poverty, but getting rid of the giant monobloc apartments in Sheffield likely wouldn't be opposed too much because Brutalism's never much been liked and there's plenty of examples.