This isn't true at all - city centres often have protection for their historic iconic buildings. In somewhere like central London the majority of buildings are reused iconic buildings. You'll find McDonalds locations in Georgian town-houses for example.
In your example, the McDonalds had to make do with the existing building which they aren't allowed to change. In the other example, non-McDonalds have to make do with an un-renovated McDonalds structure which they can't afford to change. You aren't providing a counter example, you're just talking about something different.
Things as documented by: https://twitter.com/utbaph?lang=en
A McDonalds that is built into Westminster Abbey may be part of an iconic building, but the McDonalds isn't Iconic (nor is the one on the Champs-Élysées).
Or, even, you don't have to! https://en.wikipedia.org/wiki/Oldest_McDonald%27s_restaurant "Along with its sign, it was deemed eligible for addition to the National Register of Historic Places in 1984, although it was not added because the owner objected"
Which is why you only find "reused iconic buildings" in parts of the city that are now more run-down.