I do not believe the game is nearly that balanced on a knife edge that you seem to be suggesting here. To think that any time a player chooses not to use a certain ability means that they are now playing "below their fighting weight" (to use an inaccurate analogy) and thus should be compensated somehow to get themselves back up in balance is not anything WotC has thought of or worry about.
Oh, I don't think 5e is anywhere close to balanced. But inefficient characters simply aren't as fun at the table, for me. Why would I want to stare at some unneeded trivia on my character sheet that means nothing to my character, simply because it came with my class?
Fighters get Heavy Armor proficiency-- if they don't use it, should they be able to replace it with another feature they do (and believe me, I've seen threads where some people have argued they should, and I disagree wholeheartedly)?
Yes, absolutely. The one time I had a character play a "Dex fighter", I compensated them for the unused heavy armor proficiency. (I gave them Medium Armor Mastery instead.)
Spellcasters have dozens of spells they don't use. Should they be able to strike those spells from their spells list and replace them with other features that they will?
Well, I much prefer specialized casters with just a few spells over the broad generalization of standard 5e casters, so I'm probably not the best person to ask.
Weapon-users have proficiency in hosts of weapons they will never pick up. Should they be able to replace all of those with another feature they will?
Fair point. If a player brought it to my attention that they really wanted to trade out their weapon proficiencies to specialize, I would compensate them. Most martial types want the option to be able to use found magic weapons, though, since my games have fairly frequent and randomized items.
Where does this nitpicking end? And why is it WotC's job to make it easier for them?
At some point the game would become exactly as you seem to be playing and working yours (congrats, by the way, that's exactly the way I think people should play their game, making it their own!) Where every single little feature or ability is bought piecemeal to create the character you want. Those kinds of game already exist and seem to do well... and if someone (such as yourself) decides to hammer and work D&D into a game that also works like those other games (like Gurps or Hero System)... more power to you and I'm glad it works for you! But I do not think any of us could suggest that this is the design D&D should go or even would go.
I like crunchy character building games plenty, but I've been arguing for a while now that D&D would work better
for its default playstyle if the game had limited options at the player end and encouraged fiction-based, diegetic character growth.
Basically, make standard D&D more like a Troika! or Electric Bastionland, with dozens of low-weight starting options and then unfettered abilities, gained during play, from there. No real character building at all, pick an option and play.
