In the Mural, every race in the world teamed up with Joyboy to fight Imu and his 20 regular human kings.
At this point it’s clear that Joyboy had way more help than Imu did
You have to remember that Imu can give people immortality. If the allies Imu had 800 years ago were so strong that they mattered against Joyboy, Imu would still have them around today. Since they don’t exist today, means they were worth getting rid of or just letting them die of old age for Imu