Student or Learner
Everyone of us (is going to / will) die, so we should treasure what we have.
Which one is correct?
'Be going to' is usually used as a 'immediate planned action'. So in the example, does it mean that we plan to die?
because we have some evidence for it now. It may be that we predict an event that is just about to
happen on the basis of something that we feel, see (etc.) now.
Cambridge-English Advanced Grammar in Use
Why are you claiming that "be going to" is usually used as a "immediate planned action" anyway? "I'm going to finish cleaning the house one day" has absolutely no connotation that it is an immediately planned action. However, "I will finish cleaning the house immediately" does.
Grammar books are there to guide you and help you understand how a language is spoken and why. Some rules are (almost) cast in stone, eg conjugations (I am; you are; he is, etc.) Some are suggestions about usage.
When a grammar book says "usually" or "generally", it's always a mistake to assume that it's a "cast in stone" rule.
Here is a common mistake:
Book: "In situation A, normally B is used instead of C".
Student's question: "I saw C being used in situation A. How is this possible?"
I'm not criticising. It's not always easy to tell what is actually a real "rule" and what is a "suggestion" for usage.