You can still scale SRAM by stacking it in 3D layers, similar to the common approach now used with NAND flash. I think HBM DRAM is also directly stacked on-die to begin with, apparently that's the best approach to scaling memory bandwidth too.

It'll be interesting to see if we get any kind of non-NAND persistent memory in the near future, that might beat some performance metrics of both DRAM and NAND flash.