Sauron should have won!

[Earendilyon]

Legolas, the whole passage from the Silm reads thus:

and [Sauron] wrestled with Gil-galad and Elendil, and they both were slain, and the sword of Elendil broke under him as he fell. But Sauron also was thrown down, and with the hilt-shard of Narsil Isildur cut the Ruling Ring from the hand of Sauron and took it for his own. Then Sauron was for that time vanquished, and he forsook his body, and his spirit fled far away and hid in waste places; and he took no visible shape again for many long years. [emphasis mine]

I always read this as: Sauron was thrown down (the text does not say 'killed' or 'overcome' or 'slain') in the battle with Gil-galad, Elendil and Isildur (though the last one probably did not fight Sauron till G-g. and E. were killed), as often happens in hand-to-hand battle, and then Isildur cut off his Ring. The text says at that point: "THEN Sauron was for that time vanquished, and he forsook his body". So, Sauron was only vanquished at the moment his Ring was cut off, not before.