I'm one of those who commented that giving away the most popular titles benefits the least people. What MS needs to do is give away fabulous games that everybody wanted but nobody bought - then they'd be really popular.
A very popular game by the age group of people during initial release could still be popular with the coming of age group...
My sons first Halo game was Halo Reach but sure...why would he possibly be interested in the previous games even though he loves Reach and Halo 4.
He just started playing Crackdown and loves it.
I don't think this is a difficult concept to grasp here