When D3 was released a lot of WoW players switched over to D3 while they waited for the next WoW expac.
Activision/blizzard therefore has to space out the release of their games so as not to over-saturate the market and cause players to choose to play one game over another.
By announcing that WoD is scheduled for a Fall release they have:
1) Increased the marketability of RoS since players will more enticed to switch over to a game with newer content, and
2) Increased the hype and demand for WoD so that it sells more copies when it is finally released.
So RoS will not be a failure... desperate WoW fans whose spirit has been crushed by another xpac that takes 2 years to come out will flock to D3. It does not take 2 years to make a WoW expac whose only real feature is a new continent, 10 levels and garrisons (the only features that have to be finished for the game to be released).