Yes, but everything has to have the ability to use different types of attacks, like how almost every person has the ability to both throw a rock and splash water. Some people are better at one than the other, but that doesn't mean a non-specialist isn't allowed to do something they aren't great at, even though their specialty would dissuade them from trying something else. While throwing a rock and splashing are bad examples of what we are born specialists at irl, I think it helps serve the point that the force behind an attack doesn't just come from the base power of the technique being used. Please bear with me here.
Pretend that a stone the size of your thumb has a base power of 20. If you plan on using the stone to harm someone else, it won't be very effective unless you put some force behind it to make it move (the method used to hurt someone else with the rock is like your base Atk). The same logic is applied to splashing water. If a splash was intended to get somebody as wet as possible, then you might mean to move several fluid cups of water to hit someone else. Let's pretend further that a splash that you create contains a pint of water and has a base power of 35. It's a special move, so it'll make contact differently that the rock, which was physical. if there's a pint of water sitting in front of you, it's not going to accomplish what you want until you apply the power you personally would put into a special attack to make it hit someone else. If you completely lacked the ability to make the water move, you would have a SpAtk stat of 0. This is actually replicated in the damage formula that the games use. Part of the formula divides your Atk by the opponent's Def. If you used Rock Throw and had a base attack power of 0, the game wouldn't even recognize the attack because 0 is the numerator. That fraction is multiplied to a couple of other fractions that are made up of other statistics which is then multiplied by a modifier. all that multiplication means that you end up with an incredibly small amount of damage dealt because of a little bit of addition saving the damage value from just being 0.
The point is that, in between Dynamic Punch and Aura Sphere, it's obvious that you'd want a Machamp to use the attack that his stats are better suited to capitalize on. Just because it shouldn't use Aura Sphere over D-Punch because it won't hit nearly as hard doesn't mean that Machamp shouldn't be able to use special techniques at all. It isn't "natural".
I understand your complaint, though. While there are certain Pokemon who already have specialized stats, like Breloom and Blissey, they already came with their stats, and they aren't very keen on editing former stat spreads of old Pokemon who would arguably do better with some reorganized stats. That isn't to say that there won't be a new hyper-specialized Fighting type that's everything Machamp could have been if it's Spatk stat was lowered in favor of his defense or speed stats, but...
