Considering the rate at which the console's being sold right now, reaching 50 million might be pushing it. It would depend on alot of factors, such as making a stronger first-party development front, bringing innovation to the industry (and not just adopting from the competition), as well as achieving key reliability, maybe making XBL free and not abandoning the system like they did with the original Xbox. Microsoft is still very much in the red with their 2 generations of trying to become the leading company on the market.
All you touch and all you see is all your life will ever be.