Who says Ratchet stayed on Cybertron after Predacons Rising. He did decide to remain on Earth... which later seemed kind of pointless after he was bridged back to Cybertron anyway. But his decision to stay on Earth really makes me think that he didn't stay on Cybertron after he was no longer needed there. I'm sure there are other medics who could of come back to Cybertron. Knockout is also a Medic and defected over the Autobots during Predacons Rising. Even if there are new medical bots on Cybertron there's no reason to believe that Ratchet is the only one capable of training them.