It's not just use improvements - which are pretty important - it's the fact that the past will always look more "beautiful" or "charming" to us because these buildings are no longer being built, and are therefore precious for their rarity and exoticism. Modernism looks relatively ugly to us now because it is common.
Also, we don't see the buildings the way the people did in the 19th century. For example, a lot of people seem to get sentimental about what were clearly really rough taverns and slums. For the people living then, these would be considered ugly. Since we can't be hurt by these slums anymore, they seem quaint. Dirt always looks better in the past, because we don't have to clean it. And it's kind of terrible: people turn what was ugly and mean and scary into safe decoration. Will future generations revile us for tearing down Regent Park?
The "classy" connotations of black and white also adds a layer of beauty to the old photos. Look at how "now/then" doesn't work as well when the "then" is in colour.
And no offense to Mustapha - who does a wonderful job - but when he lines up his "now" pictures with the exact position of the "then" pictures, you lose the ability to freely frame the new buildings. This usually means we are comparing properly framed old buildings to pretty random parts of new buildings. This makes the "now" look stranger and wrong.
These are just a few reasons why although I enjoy the now/then photo comparisons, I don't trust them.
An interesting series of view points leading to your argument in the last two paragraphs.
