I can think of a counter-argument to all this. I live in New York, but I have worked briefly in London. I could make a case for London that would go like this:

There are brilliant programmers in places like Italy and Poland and they feel stifled in their home countries, so they escape to London. Many of them would prefer to go to the USA but the USA's strict immigration policies keep them out. As EU citizens they have a right to move to London, so that is where they go. London is bigger than San Francisco and it is also bigger than New York. London is also much more international than just about anywhere: this place used to be the capital of an empire that held 25% of humanity. The phrase "The sun never sets on the British Empire" used to be literally true. London is nowadays the favored playground for Russian investors and Saudi princes and Brazilian entrepreneurs. The amount of money in the city, waiting to be invested, is beyond imagining. And also the amount of programming talent, drawn from all over Europe, is of the highest caliber imaginable. At some point these 2 things, the vast pool of investors and the vast pool of talent, must inevitably spark something, and when that happens the results will be great.