I do not agree that it is a plot hole, unless every possibly important unanswered question qualifies as a plot hole.
I agree that the use of eagles is an unanswered question, that might, to some, seem somewhat surprising that it was never mentioned.
My answer is that Tolkien made the artistic choice to speak of the powers of both Sauron and the Ring in poetic terms only. The last thing he wanted to do is delve into details about Sauron's Anti-craft Aircraft potential when shooting at eagles from throne of Barad-dur, even in a passing comment. He worked very hard to shield our eyes from such details.
I agree that the use of eagles is an unanswered question, that might, to some, seem somewhat surprising that it was never mentioned.
My answer is that Tolkien made the artistic choice to speak of the powers of both Sauron and the Ring in poetic terms only. The last thing he wanted to do is delve into details about Sauron's Anti-craft Aircraft potential when shooting at eagles from throne of Barad-dur, even in a passing comment. He worked very hard to shield our eyes from such details.