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?

There's always something, you forgot last gen had Minecraft which is on the same I believe.

Don't most people play Fortnite on PC? Fortnite is not a particularly demanding game after all. And the Switch is selling relatively well without GTA 5 for what is worth (not to mention GTA 5 will come to PS5 / Series X)

I wish there were no online games for console. Each generation we get less AAA single player games. Look at ps2 and how many games we got from rockstar alone. Good times

I don't think GTAV or Fortnite really drove PS4/XBO sales.

I don't think fortnite improved console sales, if anything I think it helped improve pre built PC and laptop sales as kids might have had issues trying to play fortnite on their old computers.

Splatoon 3. Nintendo will ink splat drop you into a massive colorful map the size of PUBG ones and it will be 75v75 fights over splatfest rights, single elimination. Each 75 group will have 2 minutes before hand to vote for who their commander is, and that person will have overiding mic abilities to call the shots on the map via headset with dedicated in game communications system without interference from regular players. Let the hunt begin, for the glory of the Squidgeneral!

They will include all traditional gameplay styles as well as a deeper story campaign.

Simple answer: no. Games drive console sales and every generation has it's fair share of catalizators. This generation, amongst many, many others had Fortnite and GTA:O as well.