I think pople are underestimating the valuue of the new service because they aren't used to using it. If your onward journey from London Bridge is by the Jubilee line to the west end, changing at Canada Water instead of London Bridge will only add about 3-5 minutes to your journey, and you'll have a much better chance of getting a seat.
My guess is that these trains will quickly become busy and we'll be calling for more, or for them to be lengthened to more than 4 carriages.