Monolithic 3D does provide cost reduction ! Please note that not all 3D are alike. TSV base 3D does not provide cost reduction, but monolithic 3D does, you could get more details why and how in: <http://www.monolithic3d.com/3d-ic-edge1.html>. Clearly the number one advantage of all 3D NAND which are now reaching mass production is cost reduction !, similarly CEA Letri announcment of their agreement with Qualcomm for the development of monolithic 3D states that 3D provide the benifits of dimension scaling.
We have heard these grand pronouncements from this individual before. The last I remember was on HKMG gate last ( Intel, the incumbent ) or first ( consortia of Foundries led by Big Blue ). After inevitable problems surfaced with gate first, Samsung & TSMC got burnt and had to go back to gate last and was delayed by a couple of quarters. Credibiiity gap galore.