I'm not sure how many more units they could have sold had they cut prices more. I think the main reason the 360 & PS3 have had such poor post-peak legs is because they had already sold a ton of units. By the end of 2011, their peak sales year, they had already sold over 52.6M units combined in the U.S., about three-quarters of their current lifetime tally of just under 70M. Meanwhile, the PS2 peaked in 2002, and by the end of that year it had sold only about 15.9M in the U.S., which is only 34% of its lifetime total of about 46.7M (I would have factored Xbox sales in there as well, but it died quickly after the 360 was released). The PS2 still had a lot of life left in it even after it peaked, and while sales did steadily decline over the years it had good legs thanks largely to very strong software support. Meanwhile, the 360 & PS3 took a while to really get going, and once they peaked they had already sold to the vast majority of households that were willing and able to buy a console. Further price cuts might have helped a little bit, but probably not by much. The PS2 had several price cuts following its peak, but any effects they might have had were modest, and once the PS3 was reduced to $400 that took the wind out of the PS2's sails rather quickly. Reductions to $200 might have helped the 360 & PS3 a bit in 2012 & 2013, perhaps slowing the decline a bit, but once the PS4 & XBO were released that still would've been it for them. Reducing them to $200 might have added at best several million units combined between the two for post-2011 sales, but that would have come at the expense of reduced revenues and profits. I think by time Q2 2012 rolled around and it was obvious both systems were past their peak, both Sony & MS were already working on getting the PS4 & XBO out and they probably figured it would be more profitable to refrain from heavily discounting their older systems, which weren't going to be around much longer anyway once their replacements were out. The 360 has already been discontinued, and the PS3 isn't too far behind it.
Lack of price cuts on last gen is the culprit here IMO. When you are looking at less software support and closer price ranges to currentky supported consoles of course fewer people would see value in that. Contrast that with the PS2 that continued to see price reductions as low as $99 after the release of PS3 and it's clear that the lower price point along with a substantial existing software catalog ensured the PS2 continued to sell at an impressive clip even years into the life of the PS3. Unfortunately PS3 cannot lower cost die to the higher manufacturing costs associated with its unique architecture. It just wouldn't be profitable.