Yeah but that doesn't help if it's too slow to use as DRAM

It's another level in the hierarchy. DRAM is too slow to use as SRAM, but it's denser, so it has a place. Flash is too slow to use as DRAM (or PCM), but it's denser, so it has a place. Spinning rust. Tapes. Adding another layer between DRAM and Flash to take advantage of its unique trade-offs is not only reasonable, it's exactly what we always do.