The Fellowship's True Purpose?

[Firefoot]

True, but wouldn't it have been better to use the Ring then? Then Sauron could be defeated

The problem with this statements is that Sauron never could be truly defeated as long as the Ring existed. He could come back in another couple thousand years, and then you are back where you started. The point of destroying the Ring was (I'm paraphrasing) to take care of the problem once and for all, not just for one age of men. In addition, anything done with the Ring becomes evil and ultimately works for Sauron. Gandalf also said that it would take too long for one person to learn how to use the Ring adequately enough to be able to overthrow Sauron, and in that time Sauron would learn exactly where it was and 99% chance says he gets it back.

Maybe the new Ring-user would take a while to become corrupted. After all, Bilbo did last 60 years, and still didn't show too much signs of becoming corrupted.

Gandalf hints that this has something to do with Hobbits. Hobbits appear to be more resistant to this type of evil than Men, Elves, and Dwarves. Bilbo did not use the Ring with the intention to dominate others, which also slowed his corruption. It took Bilbo a long time, but look how long it took Isildur or Sméagol. I wouldn't want to take the chance that the next ring-bearer would be the same way. And let's say you give the Ring to someone who already has a good share of power in order for that person to weild it to defeat Sauron. Galadriel would not take it because she knew that it would just be replacing one dark lord for dark and terrible queen. Gandalf pr Elrond would not take it either. Another example of hobbits resisting evil is that Frodo lasted 17 days with the shard of the Morgul blade whereas Gandalf says he knows many strong men that would not have lasted nearly that long.

He believed that strong Men wouldn't be corrupted, so he disregarded the "second Sauron" thing.

Isildur was strong.

His main goal was to defeat Sauron, and whatever happens after would happen.

The whole problem with this is that what happens after this would be bad, almost guaranteed. If it hadn't been for divine providence and Mt. Doom, the Ring wouldn't have been destroyed. There might be no stroke of divine providence later on, and short of pushing the bearer into Mt. Doom there is no way of destroying the Ring. It would not let itself be destroyed while Sauron existed, and as long as the Ring existed Sauron existed. So if the Ring was kept around, even if it was guarded under lock and key, Sauron would probably get it eventually if it wasn't destroyed.