Alma does not mean to say that a high priest will retain his priesthood without regard to righteousness. Rather, his explanation points out that the priesthood was designed to be eternal, all those who are ordained unto this priesthood are made like unto the Son of God, abiding a priest continually (JST Heb 7:3, see also Heb 7:17). It was not meant to be tried and then rejected. Once a man is called to this high and holy calling, there is no going back—at least not without severe punishment. The Lord warns, whoso breaketh this covenant after he hath received it, and altogether turneth therefrom, shall not have forgiveness of sins in this world nor in the world to come (DC 84:41). This punishment is so severe because to reject the priesthood of the Lord is to make a mockery of God’s great power and benevolence to the children of men. It also makes a mockery of the eternal nature of the priesthood.