It does not matter how much the sustainable agriculture can support, nor do we know the answer to that question, because we will not have the simple luxury to change over. These unsustainable systems will not slowly degrade. They will reach a point of collapse. Not only does this mean we will not have TIME to switch over to sustainable agriculture, but the very unsustainable methods themselves reduce the sustainable capacity of the planet. Each day as ecological damage progresses the carrying capacity of the Earth diminishes. It's not some miraculous cure to switch to sustainable methods. The planet will need decades, if not centuries, to recover from the damage we have done to it already.

On top of this the most sustainable form of agriculture is food forestry as it mimics a natural ecosystem, but food forests cannot simply be grown in a season. They must be cultivated for generations to reach their full potential. That is what the Native Americans did. I have walked the remnants of those very food forests myself and they still have food in them today after centuries of neglect. After their caretakers were wiped out or displaced.

We have essentially locked ourselves in to an unsustainable system. It would take a huge amount of time and a huge amount of effort to begin to right the course and even then it may not be enough to prevent a global famine. This isn't a question with an easy answer.