I wonder if people "centralize", i.e. gather, around cities to enjoy the more and varied (decentralized, even?) opportunities afforded. Many rural areas force centralization of, say, one grocery store or one hardware store. There is generally one town center, whereas in larger metropolitan areas one can choose from many, many different social circles, markets, and to some extent even governmental centers. That strikes me as a big win for decentralization.

Even your example of git seems counter to your point— can you imagine if all version control ground to a halt whenever GitHub went down?

I realize this is off-topic, but it is an interesting thought.