That's my point,ps5 would be backward compatible, but the ps4p wouldn't be forward, there by ps4 would have had 6 years, but the ps4p would only be 3.A 2019 PS5 could be in the same "ecossystem" by having backwards compatibility and running PS4 games at PS4 Pro specs. It's just that the PS4/Pro wouldn't be forward-compatible with PS5 games, which makes sense because in 2019 the platform will be 6 years old.
The platform as a whole would be 6 years long, but the ps4p is a mid gen machine, not just a slim or refresh.
When you buy a slim/refresh, you know that it's lasting as long as when the original came out.