You´re right about the extremophyles living at the hydrotermal vents, but these ecossystems probably constitute only a small percentage of the sea bottom.
Sorry for not being clear. Yes, Hydrothermal Vents only occupy a small percentage of the deep ocean floor, but support the largest percentage of marine life at those depths. Also, the marine life in the Hydrothermal zones is not necessarily extremophilic, because the vents create a more normal living environment (nutrients, temperature etc...) even at those great depths (this is what shocked scientists when they where discovered).
So most marine life lives in relative "shallow" waters, and than at depth with help from vents.... In the case of A447, they were basically in between, in a dead zone....like out in the middle of some desert, or on the top of a mountain. Keep in mind most of Earth is ocean, so there is plenty of empty space....furthermore, at that depth, with no added heat, the temperatures are low, which inhibits bacteria growth and decay.
Not much there...