I disagree, the fact that the Ring needed to be destroyed wasn't known until after the fact that Isildur cut the Ring from Sauron's finger and Sauron reformed.
Sauron was a Maia, who was not bound to the laws that Gandalf and the other istari were about a physical form, he had enough power to reform whether he ever created the Ring or not. So, why would the fact that he could reform after losing the Ring indicate the Ring was not destroyed? Now once he created the Ring, his power was bound to it (or "in rapport" with Sauron as Tolkien describes in a letter to Milton Waldman), so when the Ring is destroyed so would Sauron and his power would be gone, but when did Sauron figure this out? The bond between the Ring's power and Sauron's could have just been kind of like a side effect, or an unintended consequence. Hindsight bias makes things as clear as mud.
