Hi Mike,
The reason <surveypage> uses the next button is to handle pages with multiple questions, and to provide a user interface that allows the participant to navigate back to previous pages if they wish to change an answer. For cases where you are presenting one question at a time with forward-only navigation, the <trial> or <likert> element would be the way to go.
I did some thinking about whether it would make sense to provide an option for automatic navigation on a survey page, but there are some problems. The feature would only make sense for a page that had a single single question with radiobuttons, dropdowns, or listboxes. It wouldn't work for any of the other question types such as checkboxes, textboxes, sliders, because with these controls, there's no discreet event that indicates the participant has completed their response. Even then, I'm not sure we could support allowing participants to select a response using standard keyboard navigation because again, there's no discreet event that indicates the user is finished navigating through the different options using the arrow key. At the end of the day, the best design for something like this is going to look pretty similar to <likert> feature, which presents buttons that that participants press to move to the next page.
-Sean