They'll probably have item sets increase item level by 10 every player level after 50. This would mean all the current best end game items would be replaced at level 59ish, just in time for the new end game. Going from i50 straight to i130 at level 51 would mean that new players who won't do the old level 50 end game grinding would be demolished by the new zone mobs because they would have to balance the mobs around the gear you get from the zone/quests/vendors. So a gradual increase makes sense, but eventually even the BIS characters would replace their old gear as they prepare for the new end game and upon dinging 60 everyone is at the same point once again.