Man-made objects can sometimes be a glory and a bane to behold. However, you are correct in observing how over ambition is an interesting thing. These glories are not just London’s alone but look at some of our other European neighbours. Barcelona’s Sagrada de Familia is a monument in size, scale and ambition but a century after its conception it still remains a work in progress. Lamenting the idea of what could have been sometimes leads to a bit of desensitising for what we already have. Wembley is a symbol of completed ambition but it seems a little tame to those abandoned plans you mention.