I agree. The horizontal momentum should not be equal to your dash otherwise MK could kill you with Fairs to death and sonic will be crazy.
You're mistaken about this.
In Melee, characters always retained the speed from their dash when they first jumped. However, it worked in Melee so that you had a certain amount of aerial resistance, which very soon slowed you down to your usual maximum horizontal aerial speed. This effect is very visible with Melee Link or Falco while SHFFLing: you can see them take off with the same momentum but then quickly decelerate. With Captain Falcon, though, for example, he had very low resistance and so wasn't slowed down significantly, which was why he could cover such huge horizontal distances even in short hops. Notice that Melee Fox, who runs almost as fast, cannot jump
nearly as far as Captain Falcon, due to his high aerial resistance.
You're forgetting that making characters take off with their running momentum doesn't magically remove their maximum horizontal air velocity. If MK's dash was faster than his max airspeed then he would either:
a) If Brawl worked like Melee where there is a certain level of aerial resistance, MK would quickly decelerate to his usual maximum airspeed.
b) If Brawl arbitrarily limits your speed (as I think it does, the physics system being so generally awful) then MK would simply take off at maximum aerial velocity, being prevented from going any faster.
I hold that maximum horizontal airspeeds should be increased, and that you should retain the momentum from your dash. Jigglypuff (who, I think moves faster in the air than while dashing) would soon accelerate to maximum aerial speed (so (s)he wouldn't be nerfed as has been suggested), and everyone would feel much smoother to control. Brawl+ characters still feel unnacceptably stiff and awkward in the air without this hack.