oh thats true.
well yeah... 'variety' can only be achieved by the niches the guns fill also the art used in their design. but if you take that away its all basically either better, worse, or the same as the default gun. guns that are objectively worse than others are not worth using against better guns, and better guns are worth using more, etc. balance is pretty important because it kills the usage of almost all other guns except the best one
u need to stop talking in absolutes cause what you're saying is provably false
a very obvious example is a shotgun. shotguns would be better than the default gun at certain ranges but worse at longer. due to its longer refire time though, it fares slightly worse than default gun at high mid range cause you can predict and dodge individual shots more easily while spamming your own back.
other factors to consider include
how the gun fires (burst fire for easier leading, full auto for large crowds or multiple enemies, shotgun spread compensating for bad aim at mid-short ranges, sniper for very high damage at any range but punishable if badly positioned/user has bad aim),
recoil/screen shake,
knockback,
projectile visibility (magnum bullets vs sniper bullets in how much you feel like backing the forget up + how easy it is to figure out where the enemy is).
on top of that the model of the gun itself can come into play - how much it obstructs your screen, where exactly do the bullets come out from, how visible it is from a distance or through walls.
and i didnt even start talking about sound design, special projectile properties (brick damage, bouncing, entry, etc), or area of effect damage
there's no benefit in being reductionist (comparing every gun to default gun/talking about gun design solely using aspects present in default gun) - any critique around it is essentially worthless (assumes a standard when the standard itself isnt well defined or comprehensive). even though you can use these arguments against this gun in particular, using it in general is bad as its only purpose is to make yourself feel like you're being insightful or helpful, while in reality you're doing the equivalent of using big words to sound smart