Price point would be the only reason now
You can never underestimate the power of a low price point. It helped PS2 sell over 155M units. My guess is the PS4 will be $199 by the time PS5 launches. Maybe even $179. And the Pro will be $299. The PS5 is probably going to be $150-$200 more than that. Budget shoppers will buy the PS4 or Pro, since they both will be supported for another 2-3 years after the PS5 launches.