Have you seen what modern plant farms look like?

Massive water usage, lowering river levels drastically. Some don't even reach the ocean anymore (see the Colorado River, from Wikipedia: "Since 1960, the Colorado has typically dried up before reaching the sea, with the exception of a few wet years."). Aquifers are running low (here's some info on that from 20 years ago: https://pubs.usgs.gov/fs/fs-103-03/).

Pesticides, herbicides and fertilizers.

Massive land use for mono-culture farms, displacing whatever wildlife and native plants were present before the farm. Add more farms and you'll have less wildlife.

Everyone switching to plant-based diet sounds nice, but is really not going to fix much. Unless you can change farming to not be so resource intensive? There are just too many people.