To enter the kingdom of God we must become spirit:
John 3:5 Jesus answered, "Most assuredly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God.
John 3:6 That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.
John 3:7 Do not marvel that I said to you, 'You must be born again.'
At the return of Christ, a person goes from a physical body (flesh) to a spiritual one. At that time those who are alive will experience a death of the natural body.
1 Corinthians 15:35 But someone will say, "How are the dead raised up? And with what body do they come?"
1 Corinthians 15:36 Foolish one, what you sow is not made alive unless it dies.
1 Corinthians 15:44 It is sown a natural body, it is raised a spiritual body. There is a natural body, and there is a spiritual body.
Even though there are some that will not be dead at the return of Christ, the mortal and corruptible body that they have will need to be discarded so that they may inherit the new glorified immortal body.
So even though this happens in an instant, those that are alive at His coming will also die.
1 Corinthians 15:51 Behold, I tell you a mystery: We shall not all sleep, but we shall all be changed—
1 Corinthians 15:52 in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised incorruptible, and we shall be changed.