[Dylan16807]

> Because DDR4 is electrically different and memory controllers are all on-die.

Them being on-die is exactly why you don't need a socket change to take full advantage of DDR4, since they directly showed a socket can support both at once. Unless you're particularly worried about people trying to buy a new motherboard for their existing CPU, but who does that? You can tell them no without blocking CPU upgrades for everyone else.

> pre-Nehalem

> AM3 CPUs would have both DDR2 and DDR3 memory controllers

LGA1151 supported both DDR3 and DDR4.