The cost of desalination (thermal & reverse osmosis) remains quite prohibitive for most nations due to its high capital and operating cost [1]. Unless the world finds a scientific rationale for doing something of that magnitude (E.g. Pour water in the Sahara to slow down world temperature from rising) or a major resource discovery, it would seem counter-intuitive to desertify it on behalf of 2.5m people [2]. That is not even accounting the unintended consequence such as the Saharan desert feeding the Amazon [3], a place considered the lung of the earth. It would likely be more manageable to have that population become climate refugees.

[1] https://www.advisian.com/en-us/global-perspectives/the-cost-... [2] https://www.britannica.com/place/Sahara-desert-Africa/People [3] https://www.nasa.gov/content/goddard/nasa-satellite-reveals-...