I think its feasable. While there's the Wii Fit argument, like Phrancyze said, people only but 1 balance board. Yet, WM+ is a lot cheaper and would be needed for each controller, so each person would be buying 4. So take your 20 mil, divide by 4, and you have 5 mil people would need to buy WM+ for each of the 4 controllers. 5 mil people is roughly 10% of the userbase.
If that argument doesn't sit well, then just assume people buy 1 WM+. That's only about 40% of the Wii owners to buy ONE WM+ unit to reach your numbers.
Either way, I think its quite possible.







