Obviously Urbana's point was nuanced and requires some judgment in application.
Some notable distinctions between these two cases:
(1) The Detective's Building is much more interesting and unusual--I'd guess architect-designed, although I don't actually know the history;
(2) It is also an appropriate scale/density for that area;
(3) Finally, that McDonald's is the best it is going to be. With the Detective's Building, you have to use your imagination to envision what it could be.
That said--I would not in fact want every McDonald's of that era torn down in the long run, because in fact it is a pretty iconic design. That particular one, however, I think we can lose.
Edit: For example, this classic 1950s McDonalds was actually restored after the Northridge earthquake with public support: