Technically PS5 could end up the only traditional next gen console but that's only if Sony actually make a clean break into next generation. I'm not really sure if they will because if things workout for MS Xbox will basically be a pre-made multimedia gaming computer so when Xbox One Z or whatever they call it comes out it wouldn't really matter if it only sell 1 million units because the games for it is going to be made either way because they are UWP games that will run on consoles & Windows PCs & that would leave PS5 on it's own when it come to trying to start a new generation & it would be really hard to get devs to make a PS5 exclusive in the 1st year when they can just make a game that run on everything from the Xbox One/PS4 on up. If MS is successful with the generation-less console model when PS5 come out it will look like Sony is asking devs to make games twice while MS is letting them make the game once.
This is why I think PS5 will only be PS5 by name if it comes out in the next few years putting a pretty coat of paint over PS4 games & if they are planing to make a clean break into next generation they will be better off waiting as long as they can until people are bored with the PS4/Xbox One games & make a radical change with the PS5 that separate it from the PS4 , Xbox One & PC.
I definitely think Microsoft is gonna end up establishing some sort of precedent in contrast to Sony when it comes to backwards compatibility and the relationship between generations.
If Sony still enforces distinct divisions between PS5 and PS4 games or, God forbid, the PS5 isn't even backwards compatible with PS4, I think you're actually STILL going to see developers basing PS4 and PS5 versions of games off the same code base. I think no matter what at this point the PS5 is going to be technically a PS4 with a new CPU and all the other specs jacked up, there's no reason for Sony not to do that. It's just a matter of whether Sony (or Cerny) thinks certain optimizations and expectations about how some developers will code may make it safer comparability-wise to put a "wall" between PS4 and PS5. There are going to be cross-gen PS4 and PS5 games, just like there were cross-gen PS3 and PS4 games. The difference is instead of being developed for two totally different architectures they'll be developed for the same architecture but at different levels of power, which means you're potentially looking at the PS4 and PS5 versions of games being about as different code-wise as a game running on PS4 and that same game running on PS4 Pro. Why even sell them as different SKUs at that point?
If Sony sticks with the same architecture (which it probably will) AND has BC with PS4 games, why not just let developers make a "PS5 Mode" to enhance those BC games? At that point you sort of eliminate cross-gen games. They'll probably put "PS4 and PS5" on the box or something.