That would quickly change if people actually wanted to do it though. Social norms are just unwritten agreements - if enough people decide to dictate their novels and blog posts in coffeeshops then the default will quickly change so talking is acceptable.
I think you underestimate the resistance. The problem isn't just that people don't want to do it, it's also that people don't want other people to do it because it's annoying.
It might pass in a coffee shop, it probably won't in an airplane, it will almost certainly never pass in a library. People will try it regardless of the appropriateness of the location, because some people aren't aware or are assholes, and as a result the entire technology will get a bad rap. See google glass as an example.
People talk to each other all the time in coffee shops. If talking to your computer becomes as natural as talking to a friend, why won't it be acceptable in a shop?
People talk to each other all the time in airplanes too, and even libraries, but it turns out that people are both more understanding of people talking to other humans currently present (rather than say on a cell phone) and people are better at talking to other humans currently present respectfully than they are at talking to devices (or at least people via devices, but I think we can extrapolate).
I agree if it was already the social norm it wouldn't be a problem (same with Google glass), but it turns out that the technology being ready isn't always enough to make the social norms change.
I think part of the issue is that we're pretty good at focusing and filtering. If we're trying, we can focus on a single voice when the room is noisy. The computer doesn't know which to choose and all voices seem equally important.
