This is not obvious at all. Going 3D makes sense for memory, because these devices aren't heat-limited, but for CPUs it's not at all clear that stacking layers

This is not obvious at all. Going 3D makes sense for memory, because these devices aren't heat-limited, but for CPUs it's not at all clear that stacking layers would be a win. CPUs are limited by heat, and heat transport away from the CPU is proportional to area, not volume. Also, CPU costs are driven mostly by lithography, and if you're doubling the number of high resolution lithography steps, you aren't saving much money compared to just making larger area CPUs.