I think there is no difficulty curve because new players don't actually play through all the content from the start to the finish at its original difficulty. If the end of ARR was difficulty 1, end of HW was 2, and end of SB was 3, then a player starting tomorrow would basically go from 0 (because everyone else is overgeared for ARR and SB) directly to 3. That's not a curve, it's a cliff. If you want the game to be friendly to new players, you really can't have a continuous curve forever.