Apple artificially shortens the lifespan of its products through feature rationing to the devices it feels it wants to promote and leaving them off of the devices it wants to die out.
Siri is a perfect example
how long the iPad 3 will be supported will have nothing to do with the capability of the A5X chip, but rather everything to do with what Apple wants you to buy at the time of the iOS or feature introduction.
i bought the iPad 3 on October 3 and qualify for Apple's unofficial 30 day return period so i will definitely be upgrading to the iPad 4 with the A6X chip. the A5X is not even sufficient for the Retina combined with a graphics intensive app, so i don't see it being capable for much longer for games. it can barely play Metal Storm. the A6X will be a godsend to the Retina iPad