Doc
BoDoc Horseman
In TP a Goron throws link over a high cliff shouting 'A weak spirit is no match for the might of the Gorons'. They are clearly a race obsessed with srength and dominance, a weak species like the mogmas must have seemed beneath them so they got rid of them.
I agree the Goron's moved there for the rocks, but because it was already colonised the Gorons decided to wipe out opposition.
The Gorons are shown to have formed a patriarchal society in TP. This leads me to believe they did in the past. You say the gorons seem nice, well most could actually be, but what the leader says has to be done. If he was a bloodthirsty guy and he wanted mogmas gone, they would be gone.
The Gorons have been described as valuing strength, but it is never truly implied that they kill off the weaker species. In TP, the Hylians are living peacefully at the base of Death Mountain, and the Gorons do certainly believe that the Hylians are weaker but never move to physically attack the village. Most of the hostility shown in the game was a distrust, as the Gorons were having issues in their mines. Not because they thought the Hylians were weak.
Again, much of what you are saying is speculation. Yes, the Gorons did value physical strength but they weren't disgusted by weakness.