Yeah it is more complex it is called greed.So complex no one can be sure when he see it.The only reason i see a 6 core part to be not compatible with older mobos is power drain exceeding the power design of the older architecture.But in this case we are talking again for entire generation almost same as the previous one not being compatible with the previos architecuter mobos.
Remember the Phenom II ? Both AM2+ , AM3 and AM3+ sockets compatible CPUs.