Seagal's writing out is HOAX: proof here!
I just found the original 1991 script for Executive Decision, the death was intended almost at the same time he was shooted in the final cut. Seagal was a boost for the production, the death was also the suprise element in it. Heres the original draft:
https://indiegroundfilms.files.wordpress.com/2013/08/executive-decision-apr-91.pdf