I can't help thinking next gen will be starting slower out of the gate than last gen. we're all so fed up with PS360 last gen and so eager to move to PS4/XBX. this time around, we already have talks about nextgen, but I think people are still comfortable with current gen spec. you have many people saying it's too soon, or the gen just feel like it's starting to get good etc. mid gen refresh also helps.
so I'm guessing that PS4 will still continue to sell well even when PS5 is out. and if Sony can manage to sell PS4 for really low price like less than around $150 or so, I can see PS4 selling 10s of millions more after PS5 is out.
How people feel now is not going to be how people feel 2 years from now. Heading into current gen all the talk was about how consoles were dead yet PS4 steamrolled out of the gate, which caught everyone off guard. Look at publisher support in 2013/14 for proof. It's why we got so many cross ports and remasters in those first couple of years.
I think consumer spending habits with regards to electronics have changed. The moment PS5 launches it becomes the shiny new thing that everybody wants. I don't think there is going to be much of a market for the old stuff. All it takes is for a game like Cyberpunk or something similar running on new hardware to convince gamers "oh yeah maybe I do need a new box"