A very frightening scenario indeed.
However it omits a number of factors which will modify the suggested scenario.

First, The Nile Valley can support a maximum of 40 Million people.
Egypt's population rose to 83 Million during the 20th century because
oil export revenue enabled food and fuel subsidies.
However, Egyptian oil production peaked in 2000 and since then the
production decline has resulted in Egypt's present strained circumstances.

Second Saudi Arabia's oil production is believed to have peaked.
The water content, due to use of water to increase production, is rising.

The existence of an expeditionary army supported by Saudi Arabia is
a worry for everyone. Is this what they intend to do with the 40 million
surplus population that Egypt is trying to feed at present ?
Will this army be used with Israel as an excuse ?
Is it only aimed at Iran.

We could all just let them go at it and exhaust themselves.
It might well be to our advantage.