Voice interaction will get better, but I think it will take a long time before it surpasses touch. The think about touch is that visual icons tell the user about the expected parameters of operation. For example, an arrow is understood as a way to navigate through content. With voice, you have to know the command syntax or have some sense of the range of acceptable words. And most of us are still not that comfortable talking to machines; we haven't yet developed a standard set of patter for anything beyond basic file commands. Siri is smart enough but deviate from the path too much and she'll get stumped.