Don't underestimate the amount of work (and therefore time and money) it takes to make a title reasonable compatible for multi-platform AND several generations. At least if you plan to fully exploit the potential of the recent generation design wise. And old Rare for example always pushed the technical limits with their games to reach the best possible look and gameplay. That was one of their most famous trademarks if you ask me...and it was important. Why are the graphics always considered to be "not that important" in the platforming genre nowadays?
I imagine their game to be released sometime in 2016 maybe 2017, and it should be considered state of the art for two years from then on (as most good games do). By that time around 50 million units of PS4 can be expect to be sold...same goes for the box. People who still stick to an 13 year old console "ONLY" by that time probably wont be the right target group.
I mean... if someone tells me that the last console he bought was a gamecube, I consider him to be not very interested in games anymore. So I personally think going backwards would be counterproductive.
(Don't get me wrong...I even play the old SNES on a regular basis and still buy games for it...so this has nothing to do with my lack of love for the older generations