[quasse]

> logically the most likely outcome is that the Amazon will start to die

This seems like a very large leap of reasoning to "logically" make.

My understanding is that Saharan dust clouds carry phosphorus across the Atlantic, which nourish the Amazon.

It doesn't follow that Africa stopping the expansion of the Sahara will kill this cycle. The Sahel has historically been used for agriculture (with periods of massive drought in between) and preventing the Sahara from expanding into it does not mean erasing the rest of the Sahara.