Your theory is not simply weak, Scripture speaks directly against that heresy time and time again, pointedly, unequivocally, and as a intentional warning to the unsaved.
Man's spirit IS eternal, because that's the way God has planned and programmed the initial conditions of our creation and eternal destiny.
Furthermore, the unfallen Adam was UNDOUBLTELY eternal and would have lived eternally.
And the second tree of Eden devastates your theory......if you actually understood why.
Luk 12:4
And I say unto you my friends, Be not afraid of them that kill the body, and after that have no more that they can do.
Luk 12:5
But I will forewarn you whom ye shall fear: Fear him, which after he hath killed hath power to cast into hell; yea, I say unto you, Fear him.