Or we’d return to the meat/travel consumption patterns of the early 20th century: trans-Atlantic trips being a once-in-a-lifetime thing or once-every-few-years for the well-off, and the Sunday roast/chicken being the highlight meal of the week.

Sure ... but without a significant portion of people in the population being vegans it's a political suicide to even suggest that.

It has to start in our own kitchens.