In similar manner: "what about silicon photonics?"

IIUC fiber transceivers have become better than baseband copper differential pair transceivers for same-rack cluster interconnects.

And the main issue with them is packaging difficulty/needing to have a cheap link to the switching fabric data plane, if these benefits are to remain strong after accounting for the overhead that result from not being able to integrate the PHY on the same die as the fabric data plane. Both efficiency and cost.