[nilkn]

I'm pretty sure it's explicitly written somewhere that nobody in Middle-earth had the willpower to destroy the ring. So, basically, Frodo couldn't have done it whether he walked or flew into Mordor.

My understanding has always been that Gandalf alone understood this and necessarily was hushed about it or otherwise it would have totally destroyed morale. He let others believe it was possible to willfully destroy the ring when in fact it was not.

Gandalf was said to be the wisest of all of the Maiar. So if he had a feeling that Gollum would have a role to play, it was probably worthwhile allowing Gollum to play that role. I have no doubt that Gandalf knew that Frodo would falter at the end.