When it comes to choose preserve or destroy the old historic buildings, I believe that different people will hold different perspective to this question. In my point of view, a city should preserve the old historic buildings. Being a container of human activity, Building, obviously, is also a reservoir of human history and culture. To some extent, preserving old historic building means respecting the previous generations.
The first and most important thing is that we need to preserve our history, it is a commonsense. But the problem is How to preserve it? just write it in the book? Of course not. Every building has its own historical background, especially the old historic buildings from which we can know specifically something about the previous period of history. For example, if the buildings of Ancient Rome have not been preserved till now, how can we know about Ancient Rome's history precisely?
Furthermore, the old historic buildings of a city would accentuate a city's culture and characteristic. The Kashgar city is the best example, although a new modern city have been built near to the old one, the most attractive place of Kashgar is still the old city, which has many old historic buildings. The people who want to know about Uyghur culture, one of main culture of Central Asia, always go to visit the old Kashgar city.
Finally, preserving old historic building means respecting the previous generations. Maybe some old buildings have negative effect on the cityscape or its function, however, with the consideration of respect previous people, we have no right to destroy or substitute them for modern buildings. On the other hand, we have the responsibility of preserving them to our next generations so that they could have the opportunity to know about their predecessors tangibly.
In conclusion, the importance of maintaining the historic buildings of a city could outweigh the value of replacing them with modern buildings because of the reasons above.