GDDR5 got higher densities. How do you think pro cards can get 24/32GB on 384/512bit? Or Titan X with 12GB?
Pro cards also cost an arm and a leg, they can sink the cost of higher density RAM ICs. Seems cost structure dissapeared completely from the argument. I remember it was a thing when people raved about power efficiency and how it helps reduce BoM of GPUs.
So in one corner we will have the usual +20CM reference PCBs with 4GB of memory (8GB cards will cost the +100 USD offset price we say in 770 2GB vs 4GB at launch), in the other one you could have as tiny as 15CM PCBs and up to 16GBs. Any measure you take to shorten your PCB and card size with GDDR5/X adds to cost, and negates any cost advantage versus going with HBM/2 (we may as well see HBM cards for the mid range next year if you dont need more than 4GB), then add the power difference, the BoM difference higher power RAM makes (those little chips and gray boxes in the PCB arent for show, you know).
Also remember GDDR5 isnt particulary cheap, especially when you consider Nvidia already has to buy +1750mhz binned chips because of their design decisions.