I stick firm to my headcannon that Rainbow gave Hypno most of the energy needed to leave the spirit world,because frankly nobody else has any reason to trust him.
Oh,but Rainbow does. When Hypno said he would keep Rainbow safe during his hibernation he did. When he said he'd watch over the world he did. He kept every promise he's ever made to Rainbow,and he'll keep this one too,he'll fix the spirit world and then he'll come back.
He'll come back to Rainbow.
So Rainbow waits. Because he believes in him. Even after any other Steve that gave Hypno energy gives up,after practically everyone gives up on everything,Rainbow stands firm on this. Because damnit,Hypno has done the impossible before,and this isn't anything different.
If he said he would come back to Rainbow,he would.
And Rainbow is right.