The reason it has to be balanced post-ROTJ is because it’s kinda directly stated that it is. Anakin brought balance to the Force. Even in TROS he says, “Bring back the balance, Rey, as I did.”
Well, it may be the official narrative, but that doesn’t mean it makes sense. The prequels were clear that balance meant the destruction of the Sith, and despite being worse for wear, Palpatine was still alive and scheming and whispering in little Ben Solo’s head. Not to mention, the New Republic was pretty corrupt, ineffective, and filled with secret Empire supporters.
The point is, he toes the line of whatever the official canon narrative currently is, but if the official canon narrative is defective and contradictory, then that doesn’t mean much.