@Sentinel only Sabo reached the throne room.
Imu was a revolutionary too at one point but turned into the sinister monster he is now.
Think about it 20 kingdoms, each ruling in turn over the world. Imu took over quietly.
Sabo and Imu are parallels of the void century just like how JoyBoy and Luffy are parallels.
Imu lost the plot or something at some point. Or maybe he was never a revolutionary with good will, maybe he was a child and stayed a child and is still a child. Many theories can be speculated, but Luffy isn't necessarily fighting Imu.
It can be Sabo and Luffy too.