The nice thing is that you can depend on Infura for now, but if they ever attempt to be dishonest, you can easily switch to hosting your own node or light client. The cost of moving away from these centralized services is pretty low.
Will the ethereum light client run in metamask with no configuration? I’m afraid anything short of that is too heavy.
Extending the mail server analogy, updating the MX record does not exec a mail server.
Ideally, as Moxie suggested, MetaMask itself integrates a light client into its wallet, so that it becomes the default configuration.
So for metamask they would replace the calls to infura with a light client instead. Easy. They’re probably a year away from adoption, this year will accelerate development as it’s something lots of us want.