I think Adam died biologically because he had died spiritually. If you cut a branch off of a vine, it will look “alive” for a while. Its fruit will remain on it for awhile. But is is no longer connected to the source of life, so it withers as it dries up and decays into nothingness.
“The Spirit and the Bride say ‘Come, let all who thirst drink freely form the water of life’”.
But Paul assures us that the branch can be grafted back in, “Therefore all Israel will be saved.”(Rom 9)