I expect Nintendo will continue to grow as a hardware platform whereas PlayStation and Xbox will shrink. The brand power of Nintendo is incredibly strong and by expanding to theme parks and movies they can entice a whole new generation.
Playstation and Xbox are already over reliant on the same third party games to push their plastic boxes. So PlayStation and Xbox (together) are shrinking as a platform. PS5 most likely won't reach the PS4, unless GTAVI somehow brings back momentum. And Xbox Series definitely won't reach Xbox One numbers. Now that PS is focussing on becoming quasi third party I don't see PS6 doing better than PS5.
Please excuse my (probally) poor grammar







