I .. actually .. believe .. PS4 has a chance of reaching PS2. If Sony's estimates hold up, PS4 will have shipped 112M LTD by March 2020, leaving it only 43M short of the PS2.
My prediction from 2016 has PS4 lifetime at 120. It may end up surpass that but it'll definitely not reach PS2's 155 million as the $99 PS2 ($130 with inflation) sold like hotcakes next to the $599 PS3.
I can't see PS4's price coming down to $130 and I can't see PS5 going for more than $499.