King's Landing has always been in need of being rebuilt. It started off as a little fort on a hill for Aegon. People started building little hovels and hastily built gates and walls around it and it quickly became a city with thousands and thousands of people without the infrastructure or planning to handle the sudden growth. Because the central city planning involved with the construction of the city (or the lack thereof) was so poor, the city eventually became the ugly, inefficient city that it is now, with crooked cobbled streets that are too narrow, wooden shacks built on top of each other, and a huge sanitation problem. It's the youngest major city but has the worst sewage, so bad that Olenna always comments that you can smell **** from miles away. The pipes carry **** from the Red Keep to the river, but the pipes overflow during rains and the **** literally floods flea bottom. Cities like Old Town and the cities in Essos put King's Landing to shame in terms of architecture, wide paved roads/streets, sanitation, sewage systems, clean drinking water, market squares, and overall management of their population, etc which is pretty bad for the capital of the seven kingdoms.
Now, whoever that takes the Iron Throne can rebuild from scratch.