Check back in a couple million years, and maybe it will have evolved chloroplast production. But this seems unlikely to happen, given how advantageous it would be and how much time it's had to evolve without it happening yet.
How advantageous something would be when "fully evolved" shouldn't affect the chances of evolution, I think. The advantages of the incremental steps are the important.
No, no. I'm just saying that given the amount of time that sea slugs have been around, the usefulness of photosynthesizing without needing to eat algae, and the fact that it hasn't evolved yet, it's probably really hard to evolve.
