Vince always pushes like hell to make someone new the top star when his current ones hit their late 30s, and it's probably why he's lasted this long in the industry. He did this after the steroid trials and that horrendous Hogan/Slaughter Gulf War angle sent his business into the gutter rather than continuing to rely on Hogan and Savage and the like even though they still had gas in the tank. He did this during the early 00s after the InVasion flopped. And he's doing it now with Roman because he's smart enough to know Cena'll prefer to spend his 40s hanging out with Judd Apatow and his sexy common-law wife making easy money rather than getting beaten up for a living.
Vince's quests to make a new guy don't always hit the mark. Warrior burned out fast. Diesel fared even worse. Muhammad Hassan was supposed to be the top heel in the company, and he tanked like Hogan/Slaughter. But for all of Vince's many, many failures and bad ideas, he understands that you always need new blood to keep the ship afloat.
Vince's major issue now is that he's trying to simultaneously capture more casual fans while keeping Raw three hours long for the ad revenue. He almost assuredly can't have both. There's nothing remotely casual about a three-hour weekly show, but he's trying anyway because of all the money he's making from the deal.
As for Reigns? His new gimmick's working just fine, so there's no need for him to turn. He's getting regular heat, he sounds believable, and since the face-heel lines are blurred with him, he can believably tango with all the roster members who are over regardless of their status. WWE can work it just like the old days, where the champ had flexibility in their alignment depending on what the area needed.
Roman also won't be able to replace Cena's appeal, but there aren't any prospects in any promotion that can match Cena's goofy magnetism. But this also means Roman's liberated from Cena's dichotomy because he doesn't have to worry about alienating the younguns.*
* This is also why Hogan didn't turn heel until he was in WCW. Bischoff and Co., not only had a different audience than the WWF; they had completely different demographics of appeal. Hogan was universally despised by this group, and there were fewer kids for him to appeal to anyway.