Looking at the figures on vgchartz the original xbox sold 530k. The 360 has sold 1.57million. So clearly MS has made some progress in terms of marketshare.
If they manage a similar improvement in marketshare next gen of +1million it would mean 2.5million lifetime total. Or if it manages to triple figures like the xbox to 360 then it would be 4.5million lifetime total. I would expect they are hoping to make at least 2 million lifetime for the next xbox.
They are slowly building an audience there. When you have a game like Bayonetta it gains an extra 90k of sales from Japan for 360 it is better than not having those sales.