It's about the games that are developed for it. Let's say Game X is coming to PS5 and PS4. Can you say anything about how much the game is held back by the old generation of consoles. That this is so is clear, but how much is the question. There are two options for me:
1. Game would not run / look much better with full focus on the new generation
2. Game would run / look much better with full focus on the new generation
I just can't judge how much that holds back the developers
So one can't speak of a blanket withholding.
Sometimes the opposite happens: With CoD Ghosts the focus was on PS4, the previously released PS3 version (released 2 months before the PS4) was a port that was graphically absolutely awful. Simply terrible to look at and graphically significantly worse than the series' predecessors.
Sure, if you as a developer know you have better hardware available, you can theoretically deliver a bigger, nicer game.
There have been several developers who said that the base PS4 and Xbox One would hinder development. I guess the same thing happens with the next-gen. Especially with games that should run cross-gen.
There's actually a title that illustrates the whole thing very well. And yes, it clearly limits the developers.
I'm talking about World of Tanks on console.
That first appeared on the Xbox 360 and with the One also appear on this and later on the PS4. The 360 version was still fully supported until half a year ago because they played together with the next generation.
Compared to the PC, the game looked much worse textured and detailed. After the 360 version was discontinued recently and the developers were able to devote themselves entirely to the current gen, it was not only possible to add significantly more objects to the first revised maps but also to design them in more detail. In addition, the lighting could be improved and completely new and better textures were used.
This shows very well how dragging the load gene can slow down the current consoles.
But the support of the current generation shouldn't go as long as in this example. If Forza Motorsport or Fable appear on Series X in the next or following year, they will no longer be available for One.