Yes, Tolkien was a medieval historian, but he wasn't only that, and LoTR is not a work of historical fiction. Tolkien was also a storyteller, and in his famous denunciation of allegory he makes it clear that he prioritises the story above all other concerns. He was also a lover of myth and legend, which are inherently unrealistic forms. And, perhaps most importantly, he was a Christian, and his entire legendarium is an elaborate reworking of Christian theology.
Indeed, there is something Edenic about the Shire. The Hobbits are portrayed as innocents, literally child-like in stature, and protected from evil by the efforts of the Rangers and Gandalf. And this guarding of their innocence is presented as a fundamentally good thing.
To believe that the Shire was in fact a society built on economic exploitation, and that Tolkien meticulously figured all this out (but failed to mention it), undermines the morality of the tale. And the fact is, nothing truly like the Shire ever existed, so attempting to contrive a real-world explanation for its qualities is impossible. Yes, it was obviously influenced by Tolkien's understanding of and attitudes towards the English class system and rural life, but I would insist that it is basically a speculative creation, intended to portray what an idyllic version of what that society could look like, with exactly how it actually works left deliberately vague and dependent on some contrivances such as the Ranger guard.