I think he is leaving the same reason he tells Regina to leave. If everyone remembers who they are, they are going to be out for blood. Gold may not be in the book but they will remember who he is. Baelfire is just a side mission at this point, survival is a primary mission. Curse breaks they can come and go from SB and everyone will be all over Gold and Regina like white on rice.
I don’t think finding Baelfire is a side mission. He’s spent hundreds and hundreds of years trying to find him. He sacrificed the previous world in order to do this. I don’t think that counts as a side mission. I think the reason why he wants to leave is so that he can finally look for his son.