What I don't understand is why u say that the X360 will have reached 65% of it's lifetimes sales by the end of it's 3rd year. Why not 50% or 55%? That doesn't make it a PS2.
Putting at 65%, and since most ppl estimates it's 2008 sales to be 7-8 mill, that would give the X360 by EOY 2008 23-24 mill total and by end of gen 36 mill total. So, if we spread those 36-24 mill=8 mill on the years 2009, 2010 and 2011 (Sony won't have a new console released before Nov '11, thats why 2011 is regarded as end of this gen) we get something like 4.5+2.5+1 mill (really awful sales), which would mean that X360 is practically dead already in 2009.