The darkest of dark days in our history - and we still suffer from its impact now. Imagine how big a tourist pull our beautiful medieval city would have been. We would be a completely different City now.......
I agree with CaptainDart that a lot of the stuff was done post-war, even some that did not need knocking down.
There is still a fair amount of history there - some of it is hidden away behind more modern facades (i.e. many timber framed buildings were covered with brick in Victorian times to make them seem more up to date). Others are a lot of it is spread out so it doesn't give an impression of a historic city. Even some of the stuff that is, like Spon St, is fake as many of the buildings were moved there as part of the plan and so it isn't actually in context and therefore not really 'historical'.
I'm not sure it would have been great had we kept everything anyway - while the city centre may have looked nice and old for tourists, how well would it serve the needs of modern society? Would we have just needed to spread out to allow for all the new stuff that was needed, and then we may have ended up just being attached to the 'Greater Birmingham' area, with it stretching for here to Wolverhampton.
In my perfect world I'd have moved all the older buildings (where possible) into the area around the three spires (say Greyfriars Lane) and made that the historic quarter, keeping the character with narrow streets etc. and maybe even making some of it like a living museum, used for education linking in with the Herbert. Then tried to make Manor House Drive and Greyfriars Lane link up better and connect that route directly to a route from the train station. I'd also have kept the area north to north north east from the train station free from any tall development to give an unobstructed view of the three spires from the train station.