"If you come with me we could take the World" !!
And Imu domi reverse Harald, just like that......
Rocks was probably the strongest mortal and without any special power/devil fruit.
Forming a pirate crew full of strongest ppl on the seas......he doesn't really care if they truly devoted to him or respect him, he just want to accomplish his mission to take over the world.
His approached isn't stupid and that's simply the only way to do in the average mind.
Tries to overthrow Imu by brute force and pure military might.......back in the day, they won't know the existence of Ancient weapon, one piece and all necessary tools required to accomplished this nigh impossible task.
Feel bad for him tho, he did try his best. It's a realistic situation, you could be the strongest and most favoured to accomplish the job, but you aren't just fated to do so with the timing n'all etc