XP bonus wont make high gen people (like myself) retire.
I and many others only retire because we can't be bothered with playing the same build for ages. (Though I am trying to stay on one build for the first time (made it to level 31 so far), because there really isn't any reasons to retire any more).
There should be some other incentive than heirlooms and XP bonus, not that I know what it should be, maybe the 32+ heirloom that would be a 4th heirloom.