Gen 7 saw Kinect (25m) boost X360 sales beyond what would have normally been expected.
Gen 8 will see similar (+/-5%) XB/PS sales as Gen 7 but given that Fortnite and GTA-O are the biggest gaming hits since Wii Sports and Wii Fit, it's difficult to see how Gen 9 (XB/PS) can maintain the same sales without continued success of those IPs and/or new games on the same level.
I think it's going to be a long time before we see a 'new Fortnite'. GTA 6 online could continue the success of 5 but without a new Fortnite and taking into account the lower sales that would have resulted from a X360 without Kinect, it's difficult to see where any XB/PS growth can come from in the next generation.
What would PS4/XB1 sales have been without megahits Fortnite and GTA-O?
Don't Play Stationary 4 ever. Switch!