Any decent SSD has capacitor (enterprise) or battery backed (phones) DRAM. Therefore, a sync write is just “copy the data to an I/O buffer over PCIe”.

For databases, where you do lots of small scattered writes, and lots of small overwrites to the tail of the log, modern SSDs coalesce writes in that buffer, greatly reducing write wear, and allowing the effective write bandwidth to exceed the media write bandwidth.

These schemes are much less expensive than optane.

> One potential application I briefly had hope for was really good power loss protection in front of a conventional Flash SSD.

That was never going to work out. Adding an entirely new kind of memory to your storage stack was never going to be easier or cheaper than adding a few large capacitors to the drive so it could save the contents of the DRAM that the SSD still needed whether or not there was Optane in the picture.

> It seems like there's a very small window, commercially, for new persistent memories. Flash throughput scales really cost-efficiently

Flash is no bueno for write-heavy workloads, and the random-access R/W performance is meh compared to Optane. MLC and SLC have better durability and performance, but still very mid.