The thing is that if they make a single Cena once every 10 years then it's probably more than worth it.
The only problem is that those kind of guys are totally random, and not really someone who you can make. You really have to just do mass recruitment and pushing and just hope.
Or just cut the scripted promos out and let everyone improvise.
If Conor McGregor decided he wanted to be a wrestler instead of a fighter he wouldn't even be looked at by WWE because he's only 5'8" and 170 pounds, and if he were brought in as a nobody, he'd be horribly stifled by the scripts.
Just let it all free flow.