1. A gap is established between those retiring first team players and the next generation which takes a number of years to breach (Combined with an FFP embargo)
2. Guardiola's successor isn't as good at building a unit for the players to express themselves.
3. They get tired of Guardiola's intense training methods
4. Another team finds a better way to play and be successful.
5. Boredom
Yeah...that's what I came up with. I didn't bother with players leaving to seek new challenges, because I doubt anyone at Barça will leave because of that unless they aren't getting playing time. However, I do want to use it to make a point later down.
If we look back at the history of the decline of great sides, number 1 has been the most frequent cause. 2 is a bit infrequent since the good coach usually sticks around until things get sour. I assume it would be most observable in the career of Bela Guttman. 3 is what did in Sacchi's Milan. Somehow I think 4 is highly unlikely.
5 is the one that most interested me. If you want to draw a parallel between this Barcelona side and another other great team from the past, it would be Ajax from the 70's. They didn't stick together long enough for 1 to happen. 2 was a no show since Kovacs took over from Michels smoothly and so were 3 and 4.
What happened to Ajax is that they eventually got bored of being that good. Eventually the players wanted to leave for new challenges, and the ship was pretty much abandoned once Cruyff high-tailed it to Barça.
So, if I had to pick one, it would be 5. Then again, maybe they will stay like this for quite a while (or until Messi retires).
What do you guys think? Did I miss out some other possibilities?