Some HEMA types got together and made a gallery of fighters sparring in adjacent 5-foot squares: https://imgur.com/gallery/sT0EVJi
I think there’s two main takeaways from this:
#1: The average sword fight should expect to take place across at least a 2×2 area, and 3×3 is probably most reasonable. A five-foot square is a perfectly reasonable space for a single combatant to occupy. It’s small enough that they can easily prevent an enemy from passing through (unless that enemy is especially nimble, and there are class features and feats for this), but big enough that a friend can easily run through when no one’s trying to stop them, yet also too small for two people to stand in the same space and fight effectively, either with each other or with foes outside (with an exception for fighting back-to-back, which you can make a special rule for if you really feel it’s necessary).
But although a five-foot square is a reasonable amount of space for a combatant to occupy, it’s not nearly big enough for two of them to represent the space that a sword fight takes place in. The solution here is pretty straightforward: D&D really should have some more positioning mechanics built straight into its combat rules, not locked away in class powers.
#2: When you are five feet away from an opponent, you are not just squaring off with them. That happens 10 feet away. At five feet, you are actively in melee with them. I think D&D rules do a perfectly good job modeling this by having combatants who are adjacent to each other be unable to move away without provoking an attack of opportunity, but people often visualize this as one combatant turning to run and the other lunging for their unguarded back as they do so. The actual attack of opportunity comes because an enemy is disentangling themselves from a clash to get out, and you could make a strong argument that daggers (just like ranged weapons) shouldn’t get one.
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