Del Rio isn't the problem for ratings, although he shouldn't been jobbed out since Jan, before actually winning the title. He isn't a green wrestler and is still over.
The staleness of the show and lack of anything changing for years is the issue. Punk did get over and the PPV was great, but the casual viewer who doesn't spend there day on the internet probably didn't expect Punk to win or something interesting to happen. Add to the fact that it was brushed over the next night with Cena back as champion the next night.
Vince being gone from power like the 23rd time since 1998 and Triple H in charge for like the 5 time since 1999 aren't signs of changes either.
The show is still booked the same as it was in 2008.