It is believed that a careful attitude to old buildings is more significant than replacing them with new ones. From my point of view, we should keep a moderate approach to the issue. It is a profound idea to save historically and culturally valuable buildings, but it should not be a big deal to remove technically and morally outdated structures.
It is important to mention that a lot of old buildings are not just constructions, but meaningful architectural monuments. Due to this fact, we cannot demolish them when we are able to maintain them and keep the historical legacy of a place. However, some of these ancient buildings are not in a great technical condition, but contemporary construction technology lets us renovate and reconstruct them inside keeping their original appearance.
On the other hand, some old buildings are not valuable as a legacy at all. This means that we do not have to keep them in cities, and it is not an adverse idea to replace them with modern constructions. Moreover, new buildings should be created wisely to become new values in the future. Their exterior should adhere to the design code of the area where they are built, but at the same time, bring some new details there. This is a huge challenge for architects, but not an insurmountable issue.
To sum up, it is important to keep and maintain culturally and historically buildings. Sometimes it is necessary to renovate and reconstruct them. However, other old buildings may be knocked down to free up space for modern buildings that should fit the area.