Cross-gen is bit misleading though. There are literally no PS4 games that can't be run on PS3, it's just that they'd have to look like PS3 games.
Take red dead 2. They could easily make a PS3 version. But it would just look like Red Dead 1.
Look at the PS3 version of Uncharted 4 aka Uncharted 3.
They could make a potato mode 1886 if they wanted to.
Long gone are the days where a generational leap is so drastic in terms of what can be achieved, graphically.
All games are scaleable these days and if PS5 launch execs tell you that their launch games "Can only be possible by the power of PS5" or something like that. they are, quite simply, lying.
PC games have done this for years. There are PCs out there that are less powerful than a 360 and PCs that are more powerful than One X. Both can run plenty of the same games but with big differences in image quality.
I actually quite enjoy potato mode gaming. Once you strip a game back to it;s actual core ingredients, without the frivolity of photo-realism etc, you can really tell if it's a good game in terms of design.