Thinking like this never makes any sense.
First, it assumes that you buy into the WWE's kool aid that Cena is the best. "If Cena cant do it no one can!

"

Not only is Bryan a better wrestler than Cena but even in terms of kayfabe he also beat him for the title already, clean(!) After Warrior beat Hogan, should Warrior have never lost again since anyone being better than Hulk was impossible?
But the biggest issue is that this thinking is predicated on your opinion that its not believable or realistic, when the fact is that real sporting contests have upsets all the time, a hell of a lot more often than in WWE. Just because guy A beats guy B, that doesnt mean guy C cant lose to B and beat A. The Spurs can beat the Heat on Sunday, then turn around and get blown out by the Bucks two days later.