But I’m already thinking about light CPUs that use light instead of electricity for computation. Of course I don’t fully know how it works but it seems to be lower power and the next iteration of computation I guess before we get to room temp quantum computers.
If you get enough other benefits from going up from 2nm features to e.g. 200nm UV-C photons, then you may still choose to do so.
If you have a system clock running at 3 GHz, the speed of light limits your causal distance to just under 10cm per clock cycle. CPUs are already close to that for size and frequency, but let's say you're taking a 1 cm by 1cm silicon chip on a 2nm feature size process and replacing it like-for-like with a photonic chip with light that limits it to 200nm features — now it's 1m by 1m and can't go faster than 300 MHz, likely a lot less.
This doesn't mean it is useless — for example, there's a hope that it will reduce energy use, which is directly useful all by itself, but also means it may be sensible to move to a fully 3D structure which silicon can't really manage because of the thermal issues. Going from 2D to 3D helps a lot, might allow that 1m by 1m by 200nm (*2 thickness for insulation) sheet to be compacted to a 7.4mm cube, which then doesn't need to be slowed all the way down to 300 MHz due to causality.
All of these are much lower density than magnetic (hard drives) or electrical (RAM or flash).
I've not heard of RAM or flash having thermal issues (though I'm sure it would happen eventually), but that suggests 3D stacking is easier for storage, which would remove that potential advantage of optical.
One thing I've seen suggested for optical computing is to create optical elements (e.g. lenses, holograms) that represent the same transformations as a layer of a neural net; they're not at all space-efficient and any changes to the network requires basically replacing the whole thing, and you can do a similar thing with a suitable network of hard-wired resistors and transistors, but they're an interesting idea that I see come up very occasionally.
* Some people have weird ideas.
Among other reasons, you can create an electronic transistor in silicon by using an electrical signal to open and close a gate.
You can't really do this with light, light beams just pass through each other. And the kind of light-carrying media that can be affected by the presence of a control beam respond much slower and less effectively than doped silicon responds to voltage.
Nothing beats CMOS transistors in density.
You're clearly not using enough power for the Schwinger effect. (More seriously, there are other non-linear effects in non-vacuum optical media.)