Skyscrapers are needed in London. They're a necessity. International companies would rather do business in London than New York City. To accommodate for the steep rise in foreign companies, the Canary Wharf Group and the City of London have been competing with various projects to lure the big companies to let out their skyscrapers. It promotes healthy competition which in turn fuels our economy. A big example, JP Morgan is in advanced talks to let out Riverside South, two huge skyscrapers at Canary Wharf.
Also, TfL are going to take 70% of the office space at Shard: London Bridge and Shang ri La hotels have already signed on the dotted line to accommodate for the residential aspect.
Heron, 122 Leadenhall, Shard, Pinnacle (Bishopsgate) are all under construction. The contracts have already been drawn up and Borris can do nothing to stop them.
Edit: Furthermore, the designs of the skyscrapers in the City of London are designed to protect viewing corridors. Take the tapered design of 122 Leadenhall for example.