The world title is in a decline because it's been fully solidified as the trial run/consolation prize for people who don't have the level of star power of those competing for the WWE Title. Some of these guys are just glorified midcarders/upper midcarders but WWE won't let them just BE midcarders. The World title only exists to inflate certain guys' importance. They're not good enough to be in the main event, but WWE still wants to push them as "stars", so they give them the World Heavyweight Title to make them look like a big time player even though there may be 3-5 different storylines at any given time that might be more important than the World Title feud.
I like Alberto Del Rio. He's more over and charismatic than people give him credit for, but for whatever reason, he just isn't taking as a main eventer. Personally I think the World Title should be laid to rest. Rock going AWOL with the WWE Title and Ziggler's concussion proved that two major titles aren't needed. With just one world title, maybe they can start building stars instead of manufacturing them.
Right now I think WWE just expects us to care about a guy just because he has "The WCW Belt" (I'm sure some casuals still call it this despite WCW being dead for over a decade)...
WWE needs to ultimately decide who is a main eventer and who is a midcarder. No more going halfsies on it. If WWE wants someone to be a main eventer, put them in the WWE Title picture. If they don't have what it takes to be a top star, stop pretending that they are and either put them in the IC Title division, the tag division, or just make them a jobber to the stars. If the likes of Del Rio, Sheamus, Ziggler, Sandow, Rhodes, or Swagger don't have what it takes to be WWE Champion and compete with the likes of John Cena, Daniel Bryan, and CM Punk, then they don't deserve to be called World Champions or main eventers (at this time).