Earlier quoted context omitted.
Moving towards a plant-based diet would mean needing less land as we wouldn't be raising as many animals for meat. The greenhouse idea is cool, but it'd be very resource intensive. So, while we can definitely get creative with land use, we also need to consider the costs and environmental impacts.
No, it means that farmers will stop dumping so much fertilizer, herbicides and pesticides on their land chasing high yields. It means 30 bushel per acre crops instead of 100 bushel per acre crops. It means cheaper food. All good things, but land usage won't change significantly.
It would not be so bad, imho. Industrial ag might be effective (from the economic viewpoint, very destructive from the environmental one), but not as much as producers of that stuff would like us to beliveve. And if we manage to deplete our soils even further, then even industrial ag wouldn't be able to do much.
We could and should change the way we farm, while preserving comparable yields. It might be necessary to learn farming again, or invent new machinery. There are dozens of methods we could utilize (syntropic, natural, veganic farming, permaculture, food forests, nitrogen fixing plants / trees, companion planting, etc.), and new ones would be found.
https://journals.plos.org/plosone/article/file?id=10.1371/jo...
"The analysis we present here offers a new perspective, based on organic yield data collected from over 10,000 organic farmers representing nearly 800,000 hectares of organic farmland. Averaged across all crops, organic yield averaged 80% of conventional yield. However, several crops had no significant difference in yields between organic and conventional production, and organic yields surpassed conventional yields for some hay crops."