You've made interesting points, but I think the most important point here isn't one of biology but of moral philosophy: animals presumably outrank plants in terms of moral standing.
Torturing a person is presumably worse than torturing a mosquito. A person has more capacity for pain than a mosquito, and in turn the mosquito has more capacity for pain than does a heap of sand.
I figure plants lie further down the ranking than mosquitoes, for the reasons you've just explained. They're certainly further down than farmyard animals. It would be considerable moral progress to switch from torturing billions of chickens and pigs, to torturing billions of plants (and that's ignoring that a plant in a farm environment is likely 'happier', in its own terms, than a livestock animal).
A vaguely related TED talk on how brains ultimately exist to orchestrate movement, and essentially nothing else: https://youtu.be/7s0CpRfyYp8?t=15
