I've no issues with people picking Real Madrid to win la liga - they're a great team and I can see why people feel they will repeat. But, Barca is on decline?
How is Barca on a slow decline?
Two of the best young players in the World in Alexis Sanchez and Fabregas - joined last year and will only gel better with the team this season. Villa was out for almost half a season due to a freak injury, and will be back on the team. Thiago Alacantra was in his first full season last season is expected to play even better this year. Jordi Alba (left back) - the one weakness Barca had on defense - will join the team and because youngsters like Tello and Cuenca got plenty of time last season on the field, they will only add quality to the bench.
The high level of players that are coming up from La Masia will make sure Barca won't decline all that much.
Oh...and Messi is 25 - and is getting better every year. He may not score 72 goals in the season, but he will be a better player than he was last year.