There are numerous chapters dedicated entirely to Aragorn's story (as there are for Sam and Frodo, for Merry and Pippin). I very very much disagree that he is just a side character. The Lord of the Rings is of an "ensemble cast", with no single protagonist. So while Aragorn is not The Hero, neither is Frodo, nor Sam.

I would absolutely argue that Frodo is the protagonist/hero of The Lord of the Rings. ("Protagonist" and "hero" aren't always interchangeable, but they are in this case.) Yes, other characters in LotR have their own stories and even their own arcs -- but that's true for many, even most, novels over a certain length. Many characters in LotR have their own stories with beginnings and endings, but the epic's overarching plot is that of the journey to destroy the One Ring and end Sauron's threat once and for all -- and that journey is the journey of the Ringbearer.

One of the most trenchant pieces of advice I got from a workshop instructor -- an award-winning fantasy novelist -- when I was struggling with a novel outline was "come up with an arc for all of your major characters, not just Gail [the protagonist]." Every major character in a story has something they want, has a place they start and a place they end up. Obviously stories can have multiple protagonists, but not every character who has a discernible arc is a protagonist, and not every story with an "ensemble cast" is one with multiple protagonists. (e.g., Ocean's Eleven is about an ensemble, but structurally, Danny Ocean is absolutely the protagonist.)

Story structure tools don't work particularly well as blueprints -- the major failing of the (in)famous Save the Cat! is its relentless prescriptiveness in this regard -- but they can work well as lenses and, before writing, as brainstorming tools. You need to be able to tell who your protagonist is -- and, yes, there may be more than one, but you need to understand your story structure well enough to know whether you really do have a story with multiple protagonists. And if your answer to "what does your main character want" is "oh, nothing, really, they're just fine," then you need to at least consider the possibility that there is a problem with your story, not with the general principle that stories are about characters trying to resolve problems.

> I would absolutely argue that Frodo is the protagonist/hero of The Lord of the Rings.

I would say that The Lord of the Rings is the story of the defeat of Sauron, not a story of a hobbit making it alone to Mordor to single-handedly destroy the One Ring. Tolkien could have absolutely told the story while only focusing on Frodo, and only included characters like Sam, Aragorn, Gandalf, Merry, Pippin, Legolas, Gimli sporadically, mostly when they crossed Frodo's path (such as in The Fellowship of the Ring). He very deliberately chose not to do that.

In comparison, in Ocean's Eleven, the focus is always on Danny's plan. Sure, we get to see how the others are playing their part in the plan, but it's Danny's plan, and nothing in the story happens without a direct (at lest tangential) connection with him and the heist he orchestrated. There is nothing equivalent to the defense of Helm's Deep, the conquering of Isengard, the ride of the Rohirim and so on - entire hugely important plot points that have nothing whatsoever to do with Frodo's journey.

I would also add that it's not that hard to argue that, if you were to chose a single protagonist, it should be Aragorn, not Frodo - after all, the third and final book is named for him (The Return of the King) - the only character to have a whole book named for him. He also gets the most traditional, heroic ending - he is wed to the love of his life, and he rules with a just and rightful hand over the now purified world.

How do you know he's just? And why is it so rightful? Maybe it's my dislike of Aragorn as a character but who cares about who is the king of Gondor. The return of the king is a cute name for the third book, but obviously they couldn't name it "Frodo takes the ring to Mt Doom" because that would spoil too much. The return of the king is just the gambit that's used to distract Sauron while Frodo and Sam get the job done.

It's cool that Aragorn is the descendant of the traitorous and flawed royal line of Numenor, but it would be cooler if Aragorn had just went off with Arwen to hide in a crevice somewhere and let Faramir rule over Gondor, who certainly would be more loved by his people and seems to have a decent shot at uniting houses with Rohan.

Anyways, strange women lying in ponds distributing swords is no basis for a system of government. Tolkien should have known that.