> A PostgreSQL-compatible NewSQL like YugabyteDB should cost ~$15K/mo to match top-of-the-line RDS performance.

"to match" is doing a lot of work here. It's extremely unwise that a "compatible" database will have the same performance characteristics and no performance cliffs.

> So Figma spent ~25x ($400K/$15K)

They nearly got acquired for $20B, I don't think they give a hoot about 400K if it means keeping their stack the same and getting to keep all of the existing organizational knowledge about how to keep the thing online.