Barrett still might win. what makes you think he wouldn't?
#1 ~ He's English, which for whatever reason is an offense in Vince McMahon's eyes. English wrestlers are not allowed beyond a certain level in WWE, they never have been, they never will be allowed to break out until Vince is no longer around. Vince isn't high on Barrett for that very reason, and fucks around with him for his own amusement.
#2 ~ He's in the IC title scene. He's not going to go from an IC title match (win OR lose) to the Royal Rumble winner within one month. Forget it.
#3 ~ WWE have completely dropped the ball on him since he came back. Until he got the IC title feud, he had no direction, was just having random matches, and got nothing out of it. He lost too many times since coming back and whatever credibility he had as a returning wrestler was quickly shot away and he was back to his usual level, and now that he has a direction, that direction is completely below the level he's worth, and it shows that the WWE is not willing to push Wade Barrett to the levels that we know he can and should be at.
#4 ~ In case you haven't noticed, Ryback is in the midst of the single biggest push since Brock Lesnar. Despite having only been on the roster for less than a year with this character, and having only one significant win, his push has been handled to such a degree that he has more credibility than anyone else in the company, even Superman Cena himself. That level of a push is going to be capped off with a VERY fast world title payoff, and Vince isn't going to give the Rumble to some midcard English guy he can't stand when he's so obsessively pushing this green Goldberg ripoff. ESPECIALLY after the fact that he's going to lose 3 straight WWE Championship matches in a row due to the fact that he can't win the WWE title with The Rock around the corner ready to take the belt off Punk, so after 3 defeats, Ryback is going to need momentum in a big way. Cue Rumble win, Barrett gets left in the cold once again, Ryback faces Ziggler at Mania and obviously wins.
The soonest Barrett can ever hope to touch a world title is after MITB, should he win the briefcase but he's not going to. The most likely candidate for that at this point is Cesaro, but even if they have someone else win, WWE has conclusively proven that they don't want Barrett to play any part in this company's future, or at least the head man in charge doesn't, regardless of how massive of a mistake that is.