To draw a picture, in London, you've got a population of about eight million in 1600 square kilometers--about 2000 square feet per person. Build four stories tall and have 90% of the land not covered by a building, and you've got 800 square feet per person--my share of both my home and office space. You would also have more green space than does a typical neighborhood in small town Minnesota.
In other words, while I think the private sector should be allowed to build that high if they like, there's simply no need for a building so tall that the fire department can't readily help. Central planners need to clue in to the fact that skyscrapers are monuments to vanity, not common sense.