> And yet whatever argument you use to prove that startups don't need to move from London to Silicon Valley could equally well be used to prove startups don't need to move from smaller towns to London.

Well, what if it isn't the raw number that makes a city a startup hub, but having a certain minimum amount of startups? That argument would then account for small towns not being as startup-friendly as London, but London (or Boston or New York) being an equal to Silicon Valley. Maybe the startup friendliness of a town looks more like a logarithmic graph than a linear one.