What is the significance of 1 Samuel 2:27–31?

Because Eli “scorned” God’s sacrifices and did not punish his sons for their vile behavior, the Lord says, “‘I promised that your house and your father’s house would minister before me forever.’ But now the Lord declares, ‘Far be it from me! Those who honor me I will honor, but those who despise me will be disdained. The time is coming when I will cut short your strength and the strength of your father’s house…’”

If taken at face value, this passage reflects a genuine reversal in God’s plans for the house of Eli. God’s promise that Eli’s family line would “minister before me forever” can only be sincere if God genuinely intended to have Eli’s family line minister before him forever at the time he made the promise. But it is difficult to see how the Lord could have genuinely intended this if it was absolutely certain to God at the time he made this promise (let alone from all eternity) that Eli’s family would not minister before him forever.