A building is nothing more than a shell for the most part. It's all the parts that attach inside it that make it specialized. Those extraneous city halls could easily be repurposed, as their layouts share many of the aspects of business offices. Should that not work, they could be remade into commercial storefronts, or redesigned to hold electrical infrastructure by using the building as a facade, or any other number of things. For example in England many of the older pumping stations designed to keep the fens dry were repurposed as voltage stations for electricity, old rail sheds were repurposed to shelter busses, and horse stables were turned into loft-over-shop style homes. Reuse is always better than replace when it comes to something as expensive as architecture, especially architecture using lasting materials like stone.
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.
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.