How Lightning channels will helps with scaling in practice
You would not open and close a channel for each transaction.  Instead, you would have one or more open channels at all times and find routes to the places yours want to send money.


  doesn't that encourage a very high degree of centralization due to the prohibitively large capital requirements for main hub nodes?


No.  If you make a transaction of about 50 USD, any route from Bob to Alice where all channels are greater than or equal to 50 USD (in the right direction) will do.  It does not have to go through a central hub.  50 USD is not a "prohibitively large capital requirement".  If you make a 50,000 USD transaction, it's a different story.  With lightning, the network is more centralized with larger transactions because you have fewer routes to choose from.  However, the vast majority of transactions aren't going to be very large, and therefore will have a very low degree of centralization.  They can use more routes.

Because small transactions will no longer require space in blocks, this will take significant load off the main chain.