Because London is a 2000 year old city?
The great fire of London literally reduced the city by so much that practically the entire city was rebuilt; that was only 350y ago (not making the case that you’re wrong about it being old, but 2,000 years is unfortunately wrong)
Anyway, urban design is not set in stone.
It’s interesting to look at the evolution of Los Angeles as an example of a city that already exists being radically changed, entire blocks of buildings were demolished to make downtown roads so wide.
I'm not sure giving LA, one of the poster cities for how not to do urban design, as an example, is a great idea.
Paris did the same thing, only in a much more measured fashion, in the 1760s under Haussmann and Napoléon III.
However destroying parts of a city which is thousands of years old just to increase the size of sidewalks would be to destroy historical monuments for minimal gains. If anything, making most small streets pedestrian and bike only sounds smarter.
(and i was trying to correct the notion that the city of london has not had radical changes in its urban design for 2000 years)