The primarily limit on speed in large chips like processors is NOT the transistors but the transmission lines connecting the transistors. And those are already operating at 50%-75% of the speed of light. The gain going to photonics doesn't justify the cost at this point.

Another factor: pretty much EVERYTHING in current microelectronics gets dumped in the garbage if you were to 100% switch to photonics and that's a step too far. There is relatively little in electronics that can be leveraged to photonics.

In general you need about 80% overlap with the prior major technology to justify a jump that big. This was true of tubes to transistors, and then transistors to ICs. You'd see the exact same circuit used in the prior technology being repurposed to the new one - almost verbatim. And it's that leverage that justifies the inevitable higher cost of any new technology - at least you can re-use designs, skills and concepts.