Tulipmania is no more. London’s mayor, Sadiq Khan, has rejected Norman Foster’s 1,000ft “Tulip” for the City of London, crowning an unhappy week for the architect. On the same day, his obsession with “iconic” edifices hit a wall in Paris, where his gigantic twin towers at La Défense were rejected. Khan for once agreed with Historic England and others that the Tulip was just “a lift shaft with a bulge on top” – a bulge with unpleasant blisters on its side. He did not like it at all.

For London, this was something of a surprise. Khan even deplored the Tulip’s “poorly designed public space at street level”. This makes his approval of Renzo Piano’s bleak glass cube now rising next to Paddington station the more bizarre, situated as it is in a conservation area. The mayor has previously been as meek as Boris Johnson was in the presence of London’s all-powerful developer lobby. Speculators knew that, if they just offered him a handful of “affordable” flats – usually a mere 20% off asking price – they could get any height they wanted. He is even said to favour taller towers looming over conservation areas in Notting Hill and South Kensington.

If Khan really does now see some virtue in design quality, it is a bit late. Refusing Foster seems a bit hard, given that a few blisters might have softened the edge of the widely condemned Walkie Talkie down the street. It might even have deflected attention from the outrageous 22 Bishopsgate next door, which Khan approved. Its bland bulk, second in height only to the Shard, makes the Tulip look like a Rembrandt. As for Khan’s sudden attention to surrounding street-scape, one wonders if this is a true revelation or a passing fad. No London planner has worried about public space at street level in living memory – witness the surroundings of the Shard or Vauxhall Tower, both much championed by Ken Livingstone.

‘Renzo Piano’s bleak glass cube is rising in a conservation area next to Paddington station.’ Photograph: Westminster

To be fair to Khan, his mayoralty inherited a desperate legacy. Nothing in his predecessor Johnson’s likely forthcoming premiership will outlast in public perception his London skyline. Before becoming mayor, Johnson swore he would end Livingstone’s priapic fixation with “Manhattanising” London. He pledged there would be “no Dubai-on-Thames”. As mayor he instantly reneged on this pledge. He appointed the developer-friendly Edward Lister as planning supremo and let rip.

Johnson’s particular favouritism was towards speculative towers attracting foreign money, like those now rising along the South Bank from Vauxhall to Nine Elms. He called it inward investment. By the time Johnson left office, New London Architecture was estimating some 400 buildings of over 20 storeys were under construction or approved. It was Dubai-on-Thames with a vengeance. But at least people lived in Dubai. The London of Johnson and Khan is of seething streets and empty towers.

Under the London County and Greater London councils, policy was that high buildings should be clustered, as in the City, and should not loom over the parks and the Thames corridor. Under Thatcher’s direct rule in the 1980s and then under three elected mayors, this discipline collapsed. Central London is already splattered with empty and gleaming glass erections, financed by money launderers and sovereign wealth funds and with a lack of regulation that leaves foreign cities amazed. When one day the speculators vanish, the gates are locked, the lifts rust and the windows crack, I imagine people will gaze on them, as on the towers of Tuscany’s San Gimignano, and ask, what on earth were they for?

• Simon Jenkins is a Guardian columnist