Sltldr: “They’re just throwing away money, planting trees in the desert for them to die.”
The Great Green Wall is a top down, big government intervention that has little to no local buy-in and isn’t sustainable without continued big government funding.
Not surprisingly, the funding has mostly dried up, and so has the land.
In some areas I’m sure this is true. But remember that we’re are no longer living in the historical climate. Marginally suitable for trees in the past may mean no longer suitable today. And even more so with another few decades of warming.
Smaller projects would be better to start with so we can establish what works and where. That’s why these mega-projects usually fail.
But by the same token, what is unsuitable for trees today doesn’t have to be unsuitable a decade from now. Plant growth affects the climate. Retaining moisture, stabilizing day-night temperatures, retaining topsoil, changing surface albedo, triggering cloud formation, etc.
Ideally a mega-project is thousands of small projects being attempted at once in a way that is useful even if only a fraction of them work. Those 10% of places that worked could be used as a jumping off point for further efforts in the region, and in all cases people learned valuable agriculture skills they can take with them for the rest of their lives.