Photonic technology is new and not as readily available - for now.
If it turns out to be a way of minting money, demand will rise, and the competitive process of PoW will surely result in every photonic-miner owner trying to build a bigger, faster farm, just like all the others, and then we're right back to energy.
For photonics the ratio changes significantly due to fundamental physical properties of analog computation, photons vs. electrons and so on.
Anyhow the process of the Si Photonics (and memristors/other analog approaches to AI) maturing to the same degree as digital ASICs will likely be long and gradual so this isn't a practical issue for next decade.
It will take many years for 1GB of on-chip cache to be common, but when that happens, hopefully commodity computing devices can support somewhat efficient Cuckoo mining...