The funny thing about a great metaphor is that it somehow makes it easier to draw conclusions. If Paul had only said, "A startup now can be just a pair of 22 year old guys, so they can move to a new town pretty easily, so we'll probably see more clustering of startups, not less" then you'd nod and continue on. But instead he said this about startup hubs: "the particles they're attracting are getting lighter." Suddenly you can see it and feel it: move your startup to Palo Alto! Buy real estate in Mountain View! It's so obvious!