My only question is why not launch DDR5 on server for the first year or two, then move to HEDT, and then finally onto mainstream consumer stuff?
Historically peak speed for server ram is lower than peak speed for consumer ram, due to the extra controllers and hardware checks like ecc. Currently it's cheaper/efficient to scale horizontally with more [slower] servers than it is to scale vertically with faster servers, so outside of custom built super computers the server market tends to shy away from bleeding edge technology. Stability and maturity is preferred over raw improved performance.