Aren’t we already at the point where a sufficiently large amount of RAM with a UPS just enough to save everything to SSD in case of power loss is a sufficiently close approximation to universal memory?
What the world needs is cheaper DRAM, not non-volatile DRAM.
While RAM on a modern laptop will only use a few watts at most, which is little compared to other components when under load, that still accounts for quite a big chunk of total power usage while most of the system is idle, such as during light usage and with display brightness on low.