And if you did listen to the dialogs when the JK talks to Scourge he did say Revan would have faild. He had seen it in a vision.
This is obviously mentioned in the book and central to scourge betrayal. however I recall Revan saying to him that visions arent set in stone that the future is always in motion.
They could well defeat the emperor as well given the chance Meetra was about to face the emperor who was cought by suprise at that time when she was stabed.
scourge just didnt wanted to take the risk, like all sith they are afraid in the end.