Traditional farming is not sustainable because they don't have pesticides and so once in a while your whole crop is destroyed and your whole village starves to death. Some traditional practices slowly destroy the soil over time.

Fertilizers like nitrogen (one of the largest fertilizer uses) are needed for the best yields, but modern farming creates enough as on farm to be sustainable without the addition at lower yield levels.

However when a crop leaves the field it takes away some things like sulfur, potassium, and that needs to be replaced for the next crop. I'm considering this a failure of modern transport to bring those back and not modern farming - but this should feel like a meaningless distinction.

When I say modern farming is sustainable I mean that modern farming is building up soil over the decades. While we need to replace anything actually removed with the crop, the soil is getting better year after year.