The thing is though, yes, the Shield has been existent for around 5 months, however, it is still too soon for them to break up: none of the wrestlers have been given much personality yet by creative, and it is dangerous to break them up now, as they might damage them. So, if the Shield is to stay together, they need to continue to be booked strong to retain the position they currently hold. Ryback was booked as unbeatable before getting a mega push from weakly jobber killer to WWE championship number one contender, and it didn't go over too well. At PPV's you could literally hear how indifferent crowd were compared to him in comparison to other wrestlers. The Shield, however, is currently popular, they have been a success. Now yes, it is OK for Ryback to get a win, but it should be in a lower card match, or at least not against top young incoming superstars like Ambrose, Rollins, and Reigns. And yes, the Shield members' careers wouldn't be over in the case of a single loss at WM, but what good would that do? It would be a boring, pointless, usual team of faces VS heel stable booking, just as in Team WWE VS Team Nexus and in Team WWE VS The Alliance, and so forth. And yes, the Shield will have to lose sometime, but not yet.
If The Shield lose at WrestleMania, that opens up another chapter for their career. They will be able to feud with other wrestlers. Or maybe go after Championship gold. I never mentioned them breaking up, and I don't think a loss would lead to that. I think a loss would open up opportunities for their career and more rivalries besides facing Ryback again and again.
The Shield have made Ryback's life a living hell since they debuted, it's time for Ryback to get his revenge. There has to be a pay-off to this feud eventually, and pay-offs usually always end with the face winning. It's been like that for 30 years.
And you mention how Ryback shouldn't be able to beat The Shield because their "top young upcoming superstars." But, so is Ryback. Sure, he's been there for a while as other characters, but Ryback (the character) is fairly new. Just like The Shield. The difference is The Shield are the heels, and the heels generally lose their feuds to the faces.