If you take boosts into account, then that changes things. Obviously Mence will outdamage Hydreigon in that case. Personally, I think being able to hit hard on the special side is more important than hitting the physical side hard in terms of wallbreaking. Hippo, Gliscor, Skarm, Forry, etc are all prominent members of stall and can take pretty strong physical hits. But for the special end of the spectrum, there really isn't anything other than Blissey that can universally handle special attackers (maybe SpD Jirachi, but fire attacks hurt). So if you can take down Blissey, it becomes alot harder for them to handle special threats, whereas losing Forry can be remedied between the remaining physical walls.
Really, it's a matter of being able to smash your normal switch-ins hard the first time, because your opponent won't fall for it a second. All Hydreigon has to do is kill lure and kill Blissey, and then nothing stands in its way. Salamence can get past Hippo, sure...but there's still Gliscor and Skarmory left. In any case we're arguing apples and oranges. Both can wallbreak, but neither really outclasses the other.