Within every conservation effort comes the question, is this or is it worth saving? Today's modern architecture may not be of any meaningful or long lasting endearment to ourselves, that may well be the decision of future generations. As has happened before. Did you know that in the 1800's the private owners of Stonehenge felt that it was so ancient and of no use "this assortment of useless upright stones." That they were nearly able to sell the stones and recover the land for use until members of parliament truly started debating the issue.
Citizens tend to cherish the landscape before them and their antiquities but the preservation of sites didn't really catch on until population growth and booms started to threaten the very sights and places of historic interest in which we took for granted. Imagine, simply because religion may wane, pulling down St. Paul's Cathedral simply to build a tower block or regain some open space. Consider that Stonehenge may have been sold 150 years ago. There is one argument not to get stuck in the past but it is entirely another to eradicate our cultural heritage and to know where we came from and what our ancestors built or did.
In an age of documentation and the ability to behold photographic evidence for eternity, of events, places and people, it is conceivable that we could make 3D suites of all places, statues and landmarks so there would be no need to keep the real item in place. However, until that time has truly arrived, there can be no real alternative than to keep a city or country's heritage intact, renewed and in place for future generations to enjoy. As well as profit from the positive fall out from the worldwide interest that tourism can proffer these historic antiquities and landmarks.
Indeed when it comes to protecting sites of interest we learn many valuable things through understanding the nature of conserving an area or preserving a place in time. Reinvestment from tourism activities, by-laws to reduce structural growth near the site and ensuring local populations don't have a negative effect on the area also.
Similarly if there are to be building works, then the style of such should suit the environment. There could be a housing estate sat on Stonehenge, a green open space where St. Paul's Cathedral resides or the River Thames covered over to make way for a thoroughfare. The conservation and preservation of historical landmarks should not only be consideration.