Is there any scriptural ground for believing that the pre-eminence of Christ in all things (Col. 1:18) guarantees that the saved will outnumber the unsaved?
Pre-eminence is manifestly not contingent on numerical superiority. The mass who follow a rebel are morally inferior, by far, to the few who are loyal to the rightful king. The passage cited in the question does not touch the matter of the number of the saved. It has to do with the essential glory of Christ, and His work in creation and redemption, and the purpose of God in regard to Him. All this gives Him first place before all others, angels or men. He is firstborn of all creation. He is firstborn from among the dead. God so ordered it that among all He should have the pre-eminence.
It is significant that they who praise the Lord, spoken of in the Apocalypse, are described as “a multitude that cannot be numbered,” but no such expression is used of those that are lost. But believers do well strictly to adhere to what is written and not to make statements which cannot be supported from Scripture, and which in the last instance spring from human calculations.