
Isildur took it, as should not have been. It should have been cast then into Orodruin's fire nigh at hand where it was made. But few marked what Isildur did. He alone stood by his father in that last mortal contest; and by Gil-galad only Círdan stood, and I. But Isildur would not listen to our counsel. - This I will have as weregild for my father, and my brother, he said; and therefore whether we would or no, he took it to treasure it. But soon he was betrayed by it to his death
That pretty much would have ended the Third Age right there, I suppose. Sauron would have been vanquished, the Three would also have lost their powers, and the Elves would have little reason to hang around Middle-Earth.
Or the already set 4th Age would have become the 3rd Age. XDThat pretty much would have ended the Third Age right there,
I would say Elrond did so while Isildur was off guard. At least in Unfinished Tales he was accounted “a man of strength and endurance that few even of the Dunédain of that age could equal.” (Disaster of the Gladden Fields) I do not think Elrond would want to have to deal with the might of Arnor and Gondor, nor kill his nephew. Besides Isildur was not acting crazy even though it was a reckless decision (likely not one wholly in his hands) to take the Ring.