[geofft]

The one thing you can do with such a system is to make it harder to compromise the code and get away with it. Any gossip system, public ledger, etc. (including "the blockchain", but you don't actually need something that power-hungry) can enforce that my copy of the JS is the same as everyone else's copy of the JS.

Right now it's easy to get in a network position between a single person and Gmail, without anyone else knowing, especially if you're a government, which is why a valid .google.com cert is such a valuable target. Getting in a position to compromise everyone's* copy of Gmail is much more involved, and if you pull it off, it's instantly visible to the entire world.