Right, I should reformulate; they didn't want to put in a situation where his win would have caused a HUGE backlash.
I know they did it with Rock knowing that Cena would have booed out of the building, but it's not like they expect him to get booed against whoever he fights. The Streak has been the biggest achievement in the recent history of the WWE, can you imagine Cena winning against Taker? I think people would throw shit in the ring like in the ECW days.
Cena was just going to the top around 2006, and WWE had him beat Triple H and HBK back to back at WM23. That was what I was referring to by closing the show to a chorus of boos. That was at a time when Cena wasn't cemented, and still in the process of becoming the top star. WWE isn't afraid of Cena being booed. It's become commonplace now. If they didn't give a crap back when Cena was just being cemented, and had him close the show back to back in 2006/2007, why would they care now?
Cena's been established for years, and keeping him away from Taker hasn't been because they were afraid of him being booed. The timing has just been wrong. Cena was occupied with The Rock in one way or another from WM27-WM29. At WM30, Vince clearly had other plans because he fed the streak to Brock in order to set up the next poster boy in Reigns.
In regards to there being a backlash if Cena would beat Taker. I'll answer this in two parts. The first is I don't believe Cena would have ever beaten Taker for the streak. Every streak match has been predictable since WWE started focusing on the streak around 2007 onwards. It was obvious Taker was never losing. It was so obvious every year, that it was actually a surprise that Brock beat the streak. No one even cared much prior to WM30 about this match, which only added to the surprise.
Why did Brock beat Taker though? Because Vince had his master plan of putting over Reigns. That's the reason, and we know that. In the case of Cena, Vince doesn't need to feed him the streak to make him a bigger deal. He's John Cena. Dominating Cena and Summerslam 2014 did more for Brock's character than breaking the streak did.
This brings me to whether there'd be a huge backlash if Cena broke the streak. The short answer is no, there wouldn't. I can't count how many times the mantra of "whoever breaks the streak is doomed to be hated by everyone forever" was repeated by smarks. Turns out, it actually doesn't mean anything. It was a whole lot of bullshit by smarks who bought into the corporate hype just a little too much.
Lesnar was received with indifference most of the time. After breaking the streak, and then beating Cena, he was slowly cheered by fans. The exact opposite happened with him.
With Cena, the backlash would only come from fans who don't like him as it is. We're talking the demographic that has hated him since mid-2005. Even if he managed to break the streak, nothing would change in his reactions. They'd be the same as they have been for a decade. Half the fans cheering him, half the fans booing him.
What backlash is there exactly? People overrated the streak's significance and important by buying into the WWE machine, and it looks even more foolish in hindsight. Turns out, fans didn't actually care that much, because they moved on pretty damn quickly.
This is exactly why storytelling reigns supreme. Daniel Bryan's problems with The Authority meant a lot more than the Undertaker's streak ever did. Artificial hype always means less. Organic storytelling on the other hand actually gains the fans investment.
The streak breaking was shocking, but fans ultimately didn't give a crap. How did it affect them aside from there not being a streak match every year now? It hasn't. It was a shallow spectacle that had already outstayed its significance after the repeated HBK/HHH matches. WM29 had fans cheering for a heel CM Punk to break the streak of all things.