If the eagles had flown Frodo to Mordor to destroy the ring, it would NOT have been destroyed as Gollum would not be there to bring a serendipitous end to the events. It seems reasonable to imagine that Gandalf suspected more about Gollum's ultimate role than he let on, or had some idea or hunch even that things needed to play out more organically.
By the time he had walked to Mt. Doom, Frodo was exhausted, weak, malnourished, and had carried the burden of the ring for a long time. These all contributed to him succumbing to its power.
If the eagles had flown him, none of those factors would be at play. Frodo could have still had the strength to overcome the ring's power and throw it into the fires.
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.
But as the author of the OP argues, Frodo may have become even better at resisting the ring as he made his way. The proximity to Sauron could have easily been the deciding factor rather than the hardships he faced which may have even strengthened him.
That's a good point. However, is there much textual evidence that Frodo specifically built up resistance to the ring, and not just strength of character?
I guess it comes down to the following factors that might have an effect on ring-resistance:
- strength of character/will
- strength of body
- proximity to Sauron
It's probably impossible to know exactly how much each factor contributed, but still good material for fun debates.
