The central section is only going to be complete in 2021~22, which is already at the end of the 6 year window. You're proposing, basically, that they open both the east and west extensions on day 1 along with the central section.
While theoretically possible, (KW's ION is going from preliminary design all the way to opening in 6 years, 2011 to 2017,) I don't blame them for giving a little more breathing room in the schedule. But it would certainly be nice to see them gearing up to build the extensions as the central section is being finished.