ICC lasted about 12 months before cata came along.
Dragon soul lasted about 9 months.
SoO came out in September 2013. How long do you think it will last? How many months worth of replay value do you think it has ( for example, ICC had much more replay value than Dragon soul and hence kept people playing for longer).
So far we have had no 'credible' info about next expansion. Even if next expansion is released 6-8 months after blizzcon, that's still more or less dragon soul like schedule? Will SoO hold you for that long?
When do you think next WoW expansion will be released?