GnomeWorks
Adventurer
Is every person over 100 IQ fit and athletic?
Properly modeling opportunity costs falls into that whole "game design" category.
When you have people throwing their hands into the air and saying, "screw it, don't use things that are obviously overpowered! If you do, you're a jerk!" is easier than actually trying to make the game do what you claim and/or want it to do.
If you tell a fighter he gets proficiency with every weapon (and D&D pretty much does), the fighter's player is going to sift through all the weapons and find the best one. Why? Because there's no reason not to. He has to give up nothing to do so.
And if there is one weapon that's clearly better, that might be intentional on the designers' parts. Maybe they want swords to be more common, because it... fits their image of fantasy, or whatever. So swords have a slight edge. That's fine; but we, as players, need to know that, or else we run into the whole "trap option" thing again, and that's just complete crap.
If you want to balance things, IMO the best way to do it is things like the greatsword and greataxe from 3e: one has more swingy damage, but a higher crit multiplier; the other has a normal curve of damage, but crits more often, but for less damage. There are good reasons to use one or the other; there are arguments, there is an actual opportunity cost in using one or the other.
But that kind of design takes actual work. It takes consideration, it takes a large playtest group, it takes a lot of math. You know, the kind of thing that designers are paid for... the main reason these books cost money.