I’ve seen various comments on this subreddit arguing against Sam being considered the chief hero, based on Tolkien's letter 131. While I agree Tolkien wasn’t diminishing Frodo’s heroism, I don’t agree we should cast out Sam as the chief hero only because the larger context of the letter wasn’t about heroism.
The debated section from letter 131.
Since we now try to deal with 'ordinary life', springing up ever unquenched under the trample of world policies and events, there are love-stories touched in, or love in different modes, wholly absent from The Hobbit. But the highest love-story, that of Aragorn and Arwen Elrond's daughter is only alluded to as a known thing. It is told elsewhere in a short tale. Of Aragorn and Arwen Undómiel. I think the simple 'rustic' love of Sam and his Rosie (nowhere elaborated) is absolutely essential to the study of his (the chief hero's) character, and to the theme of the relation of ordinary life (breathing, eating, working, begetting) and quests, sacrifice, causes, and the 'longing for Elves', and sheer beauty. But I will say no more, nor defend the theme of mistaken love seen in Eowyn and her first love for Aragorn. I do not feel much can now be done to heal the faults of this large and much-embracing tale – or to make it 'publishable', if it is not so now.
Unpacking this, to the argument that the section is about love, Sam and Rosie’s love story is “absolutely essential to the study of his (the chief hero's) character.” In context, and especially the sentence “the chief hero” is contained, Tolkien is explaining why Sam’s love is important to understanding his investment in the quest, and why it supports him as the chief hero. Not only his love with Rosie, but his love of the shire and the value he holds in the quiet rustic things, the preservation of them why he remains involved.
From there, to the argument that Sam is the “everyman” and not a heroic character, Tolkien isn’t only painting Sam (using the example of his and Rosie’s rustic love) as the “everyman” character, but is expressing “the relation of ordinary life […] and [heroic themes]” to support why his heroism is of extra importance. The tension of the normal and plain characters rising into heroic acts is a theme in both the Hobbit and LotR books, both unspoken and spoken by the traditionally heroic characters like Aragorn and Gandalf.
Tolkien supports this idea again in letter 184.
It was very kind of you to write. You can imagine my astonishment, when I saw your signature! I can only say, for your comfort I hope, that the 'Sam Gamgee' of my story is a most heroic character, now widely beloved by many readers, even though his origins are rustic.
I also don't consider Tolkien's use of "a most heroic character" in any way diminishing the idea of Sam as "the chief hero." He's saying it specifically to build up and encourage the "Sam Gamgee" he's writing to.
To be clear, Tolkien is not saying any of the other characters are less heroic. To that argument, I agree. But I don’t see compelling evidence that Tolkien meant anything but to build Sam, the lover of quiet and rustic things, up as the chief hero (who would normally just stay comfortable in the shire), supporting the idea that ordinary characters in life doing heroic things are the ones to be considered chief among heroes.