Anything within reasonable limits can be scaled. For example, a PS4 game can't be scaled to run on a PS2 but a PS4 game could possibly run on a PS3 with sufficient cutbacks. The concept of optimization is amazing and has only vastly improved with time.
The reason for that though is that the PS4 and One were mostly a jump in GPU power, which is comparatively easy to scale (i.e., lower resolution). Also memory, of course, but similarly you can super compress audio and lower the quality of textures, etc.
The CPUs, on the other hand, were reeeally low powered even before launch. I believe I even heard that IPC or single thread performance might have even been faster last gen; the improvement was only for workloads that benefited well from multi-threading with the additional cores. So cutting that down to work on last gen definitely would be easier.
With this next transition it's very different. The new boxes will have top of the line CPUs that are many, many times faster than last gen, while the SSDs aren't even comparable, it's a whole new level. AI, physics, loading, etc taking advantage of those can't just be nipped and tucked to run on something orders of magnitude slower. They would just have to be taken out. And if your game is built around utilizing those, that means it won't work on the older consoles, and if it
has to work on the older consoles, that means your game doesn't get made.
