Next gen consoles and Nvidia rtx io is kind of doing that with graphics textures. Textures are memory mapped but stored in nvme storage. When GPU reads a texture, it first looks in the ram, if not found in the ram, the controller talks to nvme controllers to load from storage.

If you think about it, gen4 pce nvme can reach 5GB/s doing 16k ramdom reads. The bandwidth is getting close to ddr2/3 territory. And new storage tech like 3dxpoint will have ram like access latency to improve small io perf.

You will always still need ram, but you can be more efficient at how to use the ram.