Symmetry has been a big part of D&D through most of its history, even as balance hasn't been. The Great Wheel, for instance, is symmetry on a cosmic scale. 3e's "team alignment" spells and magic items are, like, peak symmetry, living in the same edition as profound imbalance.And if none of that makes sense to add, for whatever reason, would you just arbitrarily shrink the spell list in order to serve some misguided sense of symmetry? Because it isn’t balance. 100 spells could be just as powerful as the current number, if those spells are the 100 most powerful spells. The number of spells isn’t a balance issue. It’s just symmetry. It’s no different than making the “ardent” and “battle mind” so that the psionic power source had a leader and a defender, in 4e.* Design symmetry is the enemy of good design in TTRPGs. You don’t need there to be neutral outsider descended people, just because there are tieflings and aasimar. Having fiendish blood and having celestial/angelic blood are both concepts that exist outside of D&D, that D&D is modeling. If you have a concept for fire people, you don’t have to force water people into the game to stand opposite them. You don’t need to have 8 martial classes just because the phb has 8 Spellcasting classes, and you don’t need more feats or other singular options just because you add more spells. Balance is important (up to a point), symmetry is not.
4e, ironically, broke traditional symmetries quite a bit, again, with the cosmology being a big, hard to miss example.
Some symmetry certainly shouldn't go away, since it avoids needless complexity. Like WotC has never looked back to different exp progressions for every class or different level limits for every race, even as hard as it was looking back to design 5e. It doesn't hurt 5e that the full slot casters have such similar spell progressions, and can easily combine to the MC progression. If each were as different from eachother as they are from the Warlock, would it have helped the game, or just made it more complicated? Roles, with classes of different concepts being able to fill a given role are a positive, too, since roles help differentiate PCs in cooperative play in ways other than just making them identical - roles, are even an asymmetry, in that sense. If no one wanted to play a religious character in your 1e AD&D game, you were kinda screwed, because only the Cleric had adequate healing at 1st level (the Druid's kicked in at 2nd). In 2e, Cleric or Druid could handle the responsibility. In Cleric, Druid, maybe Bard or Paladin, and definitely WoCLW. 5e, Cleric, Bard, Druid, or Paladin all work, tho healing in combat is bad. 4e, Cleric, Warlord, Bard, Shaman, Artificer, and Ardent all work, with Paladin a strong showing, as well - if someone wanted to play a non-religious warrior or a mage or whatever, but you "needed a Cleric," there was a class for that.