Very extensive, Master Eagle!
If Gondolin could be protected by immense siege weapons, Sauron, who no doubt knows of the existense of eagles, could have built giant machines for shooting things out of the air. Maybe a precursor of the Patriot missile, don't you think?
And, the destruction of the Ring doesn't mean that the war is over.
[Insert quotes here, but my book is missing]
Sure, they could have ended Sauron forever(assuming they successfully do it), but what about the Southrons? The Easterlings? The Corsairs seething for revenge? Maybe many would slink away, seeing the fall of Sauron, but many, who only depended on Sauron as a sort of united banner, would still have laid waste to the rejoicing Gondorians! C'mon - Sauron destroyed? The people of Gondor would flip over in joy, and won't realise their mistake until it is too late!