Athens is like that because the way land ownership works, from what I gather they had a lot of those apartments built by giving everyone a share of ownership in the building. So to renovate large parts of the city like what Napoleon III did with Paris or Hébrard tried to do with Thessaloniki you would have to evict or buy out so many people that it becomes impossible. All that considered the city was much better a few decades ago, less graffiti and better dressed people.
Greece is still a great place to travel to just go literally anywhere besides Athens, Mykonos, or Santorini. It would be like going to the US and seeing that Manhattan is a dump thinking it extends to the entire country.
