Take a look at the Roman Forum. Many of the ruins that you see there are 1800 to 2800 years old (the Church of Saints Luke and Martina is considerably younger). That is a very long time for any given structure to stand. Even though these are structures made of durable materials like stone and marble (which would be much sturdier than limestone, orange brick, and wood, but less sturdy than steel) thousands of seasonal cycles, rains, earthquakes, and the like take their toll. Now I can't speak to exactly how you imagine/interpret classical Rome, but I think it is safe to say that yes, it was pretty glorious.

Over time however, the demographics of the city changed, politics changed, and social trends changed. Bigger and perceivedly better buildings were being built in other parts of the city (... especially with the advent of Christendom and the Church, the city underwent big-time changes regarding what parts of the city were important, popular, and bustling.

If you look at a typical city in the United States, there is a similar sort of trend going on. In many cities' early days, the Main Street or downtown was the bustling hive of activity. With changing cultural norms and changing technologies in the mid 20th century and onward, most US cities underwent a phenomenon called suburbanization--urban sprawl. Many downtown areas have been left to decay and are considered less desirable for living and working. This is a generalization, but the trend holds true for many, many US cities.

Just something to think about.