Control over The One Ring

[Phrim]

Given that, according to Tolkien, only Gandalf could have successfully wielded the Ring to cast down Sauron, I have some comments about what Sauron actually feared from the fellowship. Gandalf himself mentions the following:

He supposes that we were all going to Minas Tirith; for that is what he would himself have done in our place. And according to his wisdom it would have been a heavy stroke against his power. Indeed he is in great fear, not knowing what mighty one may suddenly appear, wielding the Ring, and assailing him with war, seeking to cast him down and take his place. That we should wish to cast him down and have no one in his place is not a thought that occurs to his mind.

If only Gandalf himself can wield the Ring in this way, why is it implied that one of many Ring-bearers may emerge to challenge Sauron? If Gandalf was truly the only person he need fear, why was he at all concerned with anyone else?

Also, Aragorn says the following after looking into the Palantir:

It was a bitter struggle, and the weariness is slow to pass. I spoke no word to him, and in the end I wrenched the Stone to my own will. That alone he will find hard to endure. And he beheld me. Yes, Master Gimli, he saw me, but in other guise than you see me here. If that will aid him, then I have done ill. But I do not think so. To know that I lived and walked the earth was a blow to his heart, I deem; for he knew it not till now. The eyes in Orthanc did not see through the armour of Théoden; but Sauron has not forgotten Isildur and the sword of Elendil. Now in the very hour of his great designs the heir of Isildur and the Sword are revealed; for l showed the blade re-forged to him. He is not so mighty yet that he is above fear; nay, doubt ever gnaws him.

Again, why is Sauron concerned with the Heir of Elendil, if a mortal man such as he cannot possibly wield the Ring against him? Sauron had no reason to fear being bested by strength of arms alone, for his armies dwarfed the forces of Gondor and its allies. It seems that the Ring was his only weakness--and why in that case be concerned with what Aragorn does?