If people of the future consider eating meat to be barbaric, I think it will more likely be a response to the fact that meat consumption is not scalable. Think of it like Aesop's fable about the fox and the (presumably sour) grapes.

In truth, if we offer animals a good life and harvest them in a compassionate manner, the ethical arguments against eating meat are weak.

People also underestimate the capacity for plants to feel. Plants in conventional monoculture farms are much more susceptible to pets and disease. Disease susceptibility is a fairly universal response to sustained stress in animals, so it seems reasonable to me that the conditions we are keeping plants in cause them distress as well. Just because plants "think" on long time-scales using gene regulation (plant biochemistry is incredibly complex) rather than short term activation of neurons doesn't mean their experience is any less salient.