In a game where exploration is a central pillar of play, encounter balance serves as an unnecessary gatekeeper to immersion, storytelling, and creativity. This thesis relies on the idea that encounter balance, defined as having a rigid system that can output the potential difficulty of a fight depending on their level, not only fails to function in the environment 5E creates, but also hinders 5E's promise of uniting mechanics with the fictional world.
For the first point, while classes are roughly balanced between one another, avenues of subclass, spell access and selection, feat choice, magic item availability, and player taste and competency all impact how classes actually perform. When viewed through this lens, the only true balance you can hope for is a soft balance wherein no one feels particularly outshined by another player at the table. Whether 5E has achieved this soft balance is debatable; many argue that this isn't true from 9th level on, 11th or 13th level on, and some more extreme opinions even posit after 5th level (or 1st). Likewise, just as many haven't had any issue with inter-party balance. The fact that both opinions can be encountered in large numbers indicates how much the aforementioned avenues impact class balance in the game. How one person plays a battlemaster is not how another person does, and that's before factoring in the GM, the type of game it is, how much loot is given, what kinds of enemies are faced, what optional rules are used, and so on. A Rakshasa will be a very different combat if the party is composed of a Warlock, Wizard, Cleric, and Druid as opposed to a Fighter, Ranger, Barbarian, and Monk.
The second, greater point is that a focus on encounter balance hinders 5E's promise of uniting mechanics with the fictional world. What I mean by this is, encounter balance says that at X level, Y number of Z creatures will be of a certain tier of difficulty. As 5E is a game, such a system is partially required; its better to know the general "power level" of your individual monsters as a DM. But this does not mean that 5E's methods and culture around doing so are necessarily precise or good.
By arranging the world in a way that adheres to strict power level, you bring to the forefront the combat simulation of the games more so then it already it. You say that while this is a game about exploration and social interaction, its really just about combat, because the pacing of the game is based specifically on the tier-difficulties of the encounters you'll be experiencing. Since the DM is encouraged in a way of thinking that models the game as a combat simulator, parties of enemies are then built strictly to match certain metrics with narrative justification given afterwards. While this in and of itself is not a problem, the deconstruction of this is that certain parties of enemies are therefore excluded from the game not because of narrative justification, but because of their mismatch with the necessitated numbers. The level 5 party will not encounter an Adult Red Dragon and have to creatively navigate around it; they'll encounter, at best, a Young Red Dragon, if that, maybe even just a Wyrmling. The scope of what I can create is hemmed in, which is fine in certain ways, but the stories that I want to tell are partially rendered incompatible with the game itself.
More ripple effects come from this. Play culture begins to turn away from creatively using what's at hand to find ways around or over massive challenges to instead using raw mechanics in optimal ways to win against enemies that were designed to be won against. Having encounters where your party is meant to feel powerful is not a bad thing; however, when every encounter is balanced along these lines, it means that players are rarely forced to think outside the box for overcoming challenges.
Playing the game purely in this way leads to a more rote experience, reducing the scope of the game in effect. This reduced scope of play is what I mean when I say that encounter balance holds back 5E. Instead of creating new tools for helping DMs come up with creative ways that a level 5 party could beat an Ancient Red Dragon (such as with a Bard-esque arrow to a weak spot over its heart, or by finding a special gem that steals the dragon's vitality, or by giving ways a legion led by the PCs could potentially trap, restrain, and butcher the dragon), we instead get a bunch of stat blocks that show the dragon in various power stages, limiting the stories that are being told to "Can you kill this thing in a straight up fight now or later?" And while this type of story is fine, and I enjoy it, it would have been interesting if 5E embraced a variety of fantastical stories instead of just that one. The Wild Beyond the Witchlight was a great attempt at doing so. More like that, with less of a kid focus and more of an Odyssey focus, would have led to a more open and varied D&D ecosystem IMO.
Just to be clear, I am not saying that playing 5E as a combat-first game is badwrongfun. I play 5E like that often and enjoy it. What I'm saying is that 5E's focus on encounter balance gets in the way of creating more vivid and varied games because of the lack of tools and the created play culture which generally colors within the lines as opposed to outside. YMMV.
