Casual fans build strong attachments to their favs, even when they are older. Its no coincidence that even with all the effort WWE has put into bringing along younger talent that this year's WrestleMania was built around a two matches where 3 of the 4 combatants had been wrestling for 15 or more years. They knew that to draw big numbers they needed to maximize the use of veterans.
When you job out popular vets just to elevate "the new guy" fans dont buy it, it turns them off. The idea that its better to make vets put over young guys is a false premise.
For a new guy to get over they need the audience to invest and care in what they do. Turning off the audience by making them mad some new kid on the block just laid out their vet fav doesnt do that. However, a legit feud where a new guy is allowed to have significant strong moments, building to multiple matches where new guy is allowed to show off significant offense, making it look as if they "belong" in the ring with the vet, THIS is how you elevate a new guy.
Sting & Lex Luger became hugely popular stars due to the success of their feuds vs Ric Flair. However, niether ever won a major match against Flair during those initial runs. It took 2 years till Sting got a major win vs Flair, Luger never did (outside some tag matches and non title bouts). However, both had plenty of top moments in their feuds, topping Flair in interviews, in ring beatdowns, post match brawls, etc. They didnt get the title but they were presented as legit main event level talent who could win the title.
Randy Savage had great runs vs Hogan in 86 & Steamboat in 87 but didnt win many matches, lost the payoff matches, and ultimately did not triumph in either feud. He did enough to establish himself with the audience that by 88 he was World Champion.
Shawn Michaels break out moment was his loss on TV vs Flair in late 91 which set about his break from The Rockers & the switch from teen idol tag guy to HBK. After a good showing in mid card feuds vs Rick Martel & British Bulldog he challenged Brett Hart for the World Title, losing cleanly in the blowoff match at Survivor Series, then putting over Hart repeatedly on house show circuit. Yet somehow this performance established him with the fans as a legit top level contender. Hart probably never realized he was grooming his successor so soon in his run as top draw, much like Flair with Sting in 88.
Eventually, if the new guy works out he has the gravitas with fans that wins over main event level vets is acceptable and that is when the company has found its new Top Guy. If John Cena had beaten HHH or HBK in 2003 fans would not have bought it. By 07 however it wasnt so far fetched. Today we debate if he should be the one to end Undertaker's streak.
Force new guys down our throats and fans will be turned off. Build their cred through involvement with established vets, not beating them per se but portrayed as equals, and soon "The New Guy" ends up "The Main Guy"