Before the two man power trip and before HHH got hurt, the fans were slowly starting to cheer for HHH against Austin. Actually there was a steel catch match where Austin was crushing the Rock and HHH was a heel at the time and when his music hit, the fans WENT CRAZY for HHH because they thought he was going to go after Austin. As most of us know he ended up attacking The Rock and thats how the two man power trip began. I believe that may have been the night after WM17.
It was. And to this day I still reference that night whenever the topic of "worst booking decisions" comes up. He'd just had an amazing fued with Austin (culminating in the legendary 3 Stages of Hell match, which Triple H won), then had his first great match with Taker, both of which had won him some fan support. With Austin turning heel, it was a great time to elevate your hottest worker to being your top face - remember, Rock left for 3 months to go film The Mummy Returns right after that cage match. Instead they took their hottest worker, killed his momentum, made him the second banana in a stable and put the midcard title on him. They had no top face. It was disgusting.
Now, that doesn't mean they should turn Cena heel. What's going to happen? Yeah, the kids are going to start booing him...but the adults - the people that are booing him now - are just going to cheer him. What do you do then? Turn him back face? He's fine the way he is. You can't cater your booking decisions to the vocal minority that is the IWC. You have to do what's best for your company, and keeping Cena face is definitely best for the company. It's great in theory to turn him heel so the fans can get behind somebody that will avenge Cena, but the WWE doesn't have anybody that's ready to do that right now. Orton? He's not that kind of face, sorry.
While I'm responding, somebody brought up Hulk Hogan...that's a great comparison, except that it's actually a terrible one. You're right that he was at the pinnacle of his popularity, other than the fact that he was about as far from the pinnacle of his popularity as he's been in the last 30 years. He'd grown completely stale in two companies. WWF fans didn't miss him and WCW fans were quickly getting bored of the same character they had seen for over a decade. They grew tired of his politics, culminating in the ludicrous "contract clause" title loss via DQ. After the abombination that was the "Alliance to end Hulkamania" storyline and match at Uncensored, WCW basically took Hogan off TV. Nobody wanted to see him. When was the last time the WWE chose to keep Cena off Raw for weeks at a time? They didn't even do it when they had a built in storyline with him being fired. Hogan basically hadn't been seen for 3 months when he turned heel. Cena is still selling merchandise, getting people to watch, and getting great crowd reactions(even if it's a mix of cheers and boos, everybody is still reacting). So yeah, bringing Hogan up in a discussion about Cena...relevant? Not so much.