CoP: September 24th, 2004
ToAU: April 20th, 2006
WotG: November 20th, 2007
Mini-expanaions: 2009
Visions of Abyssea: June 6th, 2010
Scars of Abyssea: September 9th, 2010
FFXIV Released: September 30th, 2010
WotG missions finish: November 11th, 2010
Heroes of Abyssea: December 7th, 2010
Seekers of Adoulin: March 26th, 2013
FFXIV re-release: August 27th, 2013
They were releasing major expansions once every ~1.5 years and then dropped to Mini-expanaions (basically just 3 chain quests) and then Abyssea (9 zones that already existed with a different color balance) with six years to the next (and likely final) major expansion.
WotG missions took almost as long to finish releasing as the time between CoP and WotG.
It's also hard to overlook the role that FFXIV's development (and failure, and re-development) probably had in this. I doubt we would have gotten SoA if FFXIV v1 had been a hit, but I bet that pulling people from FFXI to work on the new hotness is why WotG dropped likely-planned content over three years and left us needing the amphetamine hit that was Abyssea.
I do also buy SE's story that developing for the PS2 was hell and they couldn't keep going, though. They didn't technically drop support until 2016, which is long after the FFXIV relaunch.