Apple Watch offers a new screen to work with. Single-hand operation is still possible. It does not match the experience before for the moment, but it will get better. Viewing it as user surrounded by devices, it will offer a clearer picture.
I think voice control (Siri here) will reduce the need for another hand to get involved in operation. Actually, we could categorize all the operations. I guess for most of those high frequency operations, voice control + wrist triggered display + customized settings would be fine for many users, if they are working well. I believe Apple has fact-based analysis on this and made their choice. How that would deliver is another story.
