A lot of people thought that we should not preserve pre-war architecture, and that's why we now have a lot of modernist and brutalist architecture in its place.

I do agree that aesthetics and fitting into the neighborhood should come into the equation somewhere: the Barbican is probably worthy of preservation, some random shitty shoebox municipal library or housing estate probably isn't.

> Why?

Because history is as valuable to humanity as memory to the human.

History, yes. The actual obsolete building housing humans, no.

So is every building history, then? Are we not allowed to knock down any buildings, since they are all part of history?

You don't need to preserve everything to remember history. If we never destroy old buildings, we'd have to abandon all our cities and find new places to build new cities, because we can't just keep living like it's the middle ages (or whatever time period you feel we need to preserve).