Other dates worth noting:

• June 23rd, 2015 - Blizzard releases Hellfire Citadel patch to Warlords of Draenor. This will be the last raid released for the expansion. Future patches will add alternate paths to upgrading gear, but no new group content.
• August 8th, 2015 - Blizzard announces World of Warcraft: Legion at Gamescom event.
  
• November 6-7th, 2015 - Blizzard announces upcoming Legion Beta at BlizzCon
  
• November 19th, 2015 - Blizzard opens Alpha for Legion to select players. Alpha continues to present date (April 18th, 2016)
  

And while I criticize Blizzard for expecting current subscribers to be satisfied for a year-long content drought... in context it will have only taken Blizzard 2 days more to develop Legion than it did Warlords of Draenor.

Draenor took 386 days from reveal to expansion launch (NOT the pre-launch event, the actual expansion). Legion will have taken 388 days.

Conclusion - Blizzard doesn’t have a clue on how to follow-through on their promise for faster expansion turnaround.