Architecture gives cities their soul, with that said i believe we should preserve old buildings but to a certain extend.
As time passes, engineers learn more about materials and the new ways to use them, as a result we can make cheaper, safer and more comfortable buildings and that is the main reason a lot of people argue for demolishing old buildings and building new ones; as the population of earth increases and as our excpectations of comfort change, we would want a house that is more comfortable and has modern technologies like noise isolation and adaptive temperature system, but the main concern of the modern homeowner is safety, we live with a lof of natural and manmade hazards that threaten over lifes on a daily basis, like tornados and earthquakes or fires, we need a house that can stand these or even midigate them from ever happening.
On the other had, old buildings have a nice and beautiful soul to them and that is why so many people try to rebuild them and make them as safe as possible while maintaining the old idea and vision, some of these efforts can be, introducing modern technologies like smoke alarms, nosie cancelling windows and new cooling and heating systems to the old house to make it more comfortable to live in, in addition we can make them safer by repairing damaged structure or by changing some of the material to modern and better ones.
At the end destroying old buildings and making new ones is the easier option, however as humans we value art and soul more than money thus we try our best to preserve what we can of these old buildings.