The characters are aware of the in-game reality which the rules reflect, and given that I won't make a character who is suicidally incompetent, there is no conflict between Optimization and Role-Playing. A character who chooses a sub-optimal weapon, merely because it is "cool", is a fool and a liability that shall not be suffered by the other individuals in the group.
Seriously, building an incompetent character is a jerk move to everyone else at the table. Don't do that. If you fail to kill the dragon because your sword only does a d6 instead of a d8, and then the dragon breathes fire and kills the whole party, then that TPK is entirely your fault and you should feel bad. There are millions of ways to build and play a character that isn't incompetent; it is not a meaningful limit on your freedom of expression.
I will say that I don't particularly agree with your tone.
However, allow me to express my take on what you said in the hopes that it may be expressed in a way that's more palatable.
I think we can all agree that being an adventurer is difficult. It's a rough life filled with danger at every turn. A lot of adventurers face the threat of violence from various monsters and ill-mannered humanoids, and there will be times when you can't just flee or talk your way out of a situation. That's when combat comes in.
Adventurers generally recognize that their job is difficult, violent, and requires a measure of combat skills to survive it for more than a few weeks. Adventurers also typically recognize that different weapons have different degrees of effectiveness. Any adventurer who knows about weapons is going to recognize that when it comes to cutting down an enemy quickly a longsword is better than a dagger.
It's also important to recognize that adventurers live in a different world than we do. In their world, reach isn't as much a thing as it is for us. In our world, the reach difference between a rapier and a dagger is big deal: in their world, it's not. Unless you're using the optional weapon speed rules, a weapon's size/weight has no impact on who will get that first swing: for us, that matters. Optional rules aside, the main factor for determining how effective a weapon is in the world that D&D adventurers reside in is the damage die (in the case of ranged weapons the different ranges are pertinent as well).
The damage die is really an abstraction, and it's frankly a rather poor one. Every weapon on the weapons table is a lethal weapon capable of killing with a single blow under the right conditions. A more accurate representation would be if they all had exploding damage dice (i.e. a max result means rolling again and adding the results together) or if damage was based on class to represent varying degrees of proficiency in arms. However, unless one is using an optional rule the die type is the most important stat in terms of how effective it is a weapon in the hands of anyone who picks it up.
Now, PCs who are just starting their career as adventurers may well not know which weapons are the best, but people who have been around for a while are likely to know this: they won't know what the damage dice are, but they should be able to get an approximation of that knowledge by realizing "that fight took longer than I thought" (less damage = more required attacks = more rounds = more time) or "he took a lot more cuts before going down" (less damage = more required attacks). If a PC is mentored in the combat arts by someone, that is probably going to be one of the lessons that is passed down.
This dominance of the damage die type is especially present at low levels and when stats that add to weapon damage are not optimized. If you have +1 damage from your strength, then a +1 average damage from an increase in die type is much larger by comparison than it would be if your stat gave you a +3 or +4. If you're talking about the difference between a d6 weapon (3.5) and a d8 weapon (4.5) then you're talking about an improvement in effectiveness of more than 25% (a 25% improvement is nothing to sneeze at). At early levels where your hit points are at a premium and where you are likely facing lower HP enemies, felling foes faster can be the difference between living and dying, and an extra point can be the difference between taking two rounds to kill an enemy or three rounds.
At later levels the one point average, two-point maximum, will fade into the background of magic weapon bonuses, improved stat bonuses, etc., so the difference eventually becomes negligible. However, it is fair to say, especially at early levels, that in the world whose physical laws are represented by the rules of the game some weapons are just flat-out better than others. It is equally fair to say that in that world people who know about weapons are likely to eschew the inferior weapons unless they know of a special circumstance that would favor those weapons (like having to conceal a weapon favors a dagger over a short or long sword).
