>>So, DRAM market is highly vulnerable. A diruptive technology could easily take majority DRAM market share. <<
The underlying assumption that the the three major DRAM vendors will sit still and watch someone try to take a majority market share is questionable at best. It is difficult to do DRAM at 1x and it is difficult to do 3D anything, DRAM or NAND. No one will come in without actual production experience and start making 3D DRAM without the Three Amigos noticing and quickly reacting. Building a fab for that ambition would be a recipe for losing billions of dollars.
Of course, the Chinese government has built large cities that remain ghost cities, monuments to foolishness. I suppose anything is possible, including building a ghost fab.
I also think DRAM will continue its scaling. Though, transition will be much slow because technical challenges are growing exponentionally and scaling of DRAM is not economically favorable due to high manufacturing cost. Much anticipated and delayed EUV should be required for advanced DRAM manufacturing below 15nm.
DRAM market is about $45 billion and recently revenue growing becomes so fast because of limited supply and less competition. So, DRAM market is highly vulnerable. A diruptive technology could easily take majority DRAM market share.
NAND vendors claim that they are building many layers of 3D NAND. However, there is no public information about cost-per-bit of 3D NAND. Based on SSD market price, there are about 10% to 30% price difference between planar-based and 3D NAND-based SSDs. So, the price difference of planar and 3D NAND components might be 15% to 50%, I guess. Please feel free to correct me if this estimation is wrong.
I think many will take issue with your claim that 3D NAND has not achieved lower cost/bit than planar NAND yet. Likewise 18 and 16 nm DRAM are already being done rapidly in succession in the same places, and 1z is already being planned. Not to discourage the consideration of SGT, but the mainstream is moving very fast.