Brice Cooper, the new Design on A Dime host, says tear em down:

But, city planners should be willing to tear down and start over, he said.
"Just because something is old doesn't mean it is great design," he said.

I don't know what buildings he's specifically thinking of, but in general I think the greater problem is architectural timidity. It's constructing buildings in styles that are supposed to "fit in," which is just about the worst mistake that can be made in architecture.

In any case, preservation is important and just as important as preservation is building buildings that will be preserved in the future because they were when they were built a product of the best, most innovative ideas of their time. Buildings must reflect their era (though the greatest seem to magically transcend era and are timeless). This is less important with residential homes, but is critical for civic buildings. Libraries, museums, and concert halls have almost always been built with great care and rightly so, because they represent the aspirations of the community and its highest values.

Read the article.