The Fall of Shiloh
First off, the Lord is ending Eli's family monopoly of the Ark "forever." But in 1 Samuel 3.30 He says: "I promised that your house and the house of your father should go on forever."
Such promises from prophets-- and such threats-- are evidently contingent. So far as we can trust them, it is not because a prophet has said them; we can trust God to fulfil what God intends, which ultimately is our good.
But how we understand God's meaning, at a particular moment, is limited by what we are able to understand... and with words like "forever," our minds just boggle.
It was not thinkable-- at first-- that Eli's family would be corrupted by their position, and lose that position, which they otherwise might have enjoyed "forever". But we've also got (2.25) "[Eli's sons] would not listen to the voice of their father; for it was the will of the Lord to slay them." That's not really thinkable to us, if we know God as Jesus described Him. But it expresses some kind of truth, for those young men did die badly, unwilling to stop grabbing the goodies. Their souls are in God's care, but their historic fate was part of the prophetic message.
It isn't that what was prophecized "had to happen." It didn't. There are several examples of people listening to a prophecy, taking the warning, and escaping the doom.
The larger purpose God intends-- required an illustration. People being as we are, someone, sooner or later, was going to take his priestly privileges for granted, abuse them, and die. Which served to give Jeremiah a precedent, to give Jesus two precedents. That what happened to Shiloh would happen to Jerusalem. "Do not trust in these deceptive words: 'This is the Temple of the Lord, the Temple of the Lord, the Temple of the Lord.... Go now to my place that was in Shiloh, where I made my name dwell at first, and see what I did to it for the wickedness of my people Israel." Jesus, in turn, alludes to Jeremiah's prediction in the course of his own warning, that the Temple of his day would be destroyed within the lifetime of some of his audience. Both predictions were fulfilled, and on the same day of the calendar.
Whether or not all this was scheduled in Eli's day, who knows? The shape of what would happen was written in our natures. People think God shares their religions. That they can bring the Ark into their battles as if God were bound by their understandings, always inclined to support their personal or national interests. It doesn't work. That's hardly the whole Message, but it's been a very hard lesson to get across.