Well, high speed SerDes transceiver only became "worth it" fairly recently (around when PCIe/SATA/SAS landed). I think this is due to switching speeds for the required SerDes logic only becoming fast enough (compared to parallel line drivers and the frequencies you can run through FR4).

DRAM consumes less power than SRAM. Far less. My understanding is, that simply changing the parameters of DDR4 towards lower latency would yield better results.