1) The movie is misleading: in the books Cirdan and Elrond don't manage to convince Isildur to go up to the Cracks of Doom. They would've had to drag Isildur against his will, and there was an army of Gondorian soldiers (not to mention the Ring) who would've objected to that!
2) Cirdan and Elrond quite possibly didn't know Sauron could come back.
They were probably just worried because it was an artifact of Sauron and therefore dangerous, and they knew that whoever held the One Ring could control the Three Rings of the Elves. But they couldn't raise too much of a fuss or that little secret might come out!
In the end it was a thousand years before Sauron began to "coalesce" again, and more than 2500 years before Gandalf discovered that Sauron had not been destroyed. The Enemy was very careful to hide his movements and work through minions.
3) Killing the King of Men is a heinous crime and not something to be done casually; the political consequences for an Elf killing a Man and the Heir of Elros are pretty mindboggling.
4) Isildur claimed the Ring as a "weregild" for his father and brother. That is a very serious thing: that meant it was the payment/restitution given him for their deaths and his loss, and symbolically equal to their lives.
5) In the real story, Elrond was Gil-galad's herald, and Isildur would probably not have considered him an equal (although in fact Elrond was Gil-galad's heir and important among Elves). Nor was Cirdan a king. They could advise, but they didn't have Gil-galad's clout among Men.
I'm sure the reason Jackson showed the Cracks of Doom in the prologue is to give us a visual idea of the Fellowship's ultimate goal, but unfortunately it made Elrond look like a doofus.