That's quite the time leap you took there with 'attitude era' fans jumping ship because Cena reached the top. They were long gone after the Invasion flopped, and WCW fans were gone long before that. Wrestling's popularity hasn't been in a severe down tick for twenty years because of John Cena, a guy who wasn't pushed until the late 2000's and didn't start becoming a belied figure until the early 2010's. If anything, despite the crowd reaction he got, he sold tickets and merchandise. You're right in thinking they need a Cena right now because he was the last big draw they had. Sure, they ran him into the ground for over ten years, but he was still a draw.
Cena could be simply stating the company edict at this point. Vince doesn't think he needs a top star anymore. Vince thinks that the WWE brand is the star, and that no performer is bigger than those initials. Sadly that line of thinking isn't panning out. While I don't think the arrival of AEW will be WWE's death knell, WWE's certainly it's own worst enemy right now and it all comes down to how the infrastructure works. Scripts are changed moments before the show starts, and sometimes during the show itself. Talents afraid to have input on their own characters, and if Jon Moxley's telling the truth, when they try they're shot down because 'Vince knows best'. They can call up all the NXT talent and hire all the writers and continuity directors they want, as long as Vince goes through and bullies and lobotomizes his vision before the camera's role, there's nothing that's going to help talent get over or the show to get better.
In a way Cena's right. WWE doesn't need him. John Cena doesn't fix the problems that WWE currently has. He's put over talent, both in the ring and on the microphone, and even that has proven to be futile.