And the thing is, it's hard to argue with the idea.
Neither the Emperor nor the Death Star had ever been a threat to the Rebellion, so, you know, fuck those first three movies. The Imperials had been able to control the Galaxy without a Death Star for a couple of decades, relying instead on fleet warfare and ground support for good old fashioned genocide. As for the Emperor, does killing the leader of a tyrannical government with a powerful and loyal army immediately end the entire conflict?
According to the majority of the books and comics set after the original trilogy, with the Emperor gone, there were hundreds of Admirals, Generals and Politicians who vied for control of the Galaxy. Without a universally accepted leader, the Empire spiraled into a civil war.
The Rebellion is still, well, a Rebellion, which means it still has to gain victory over the remaining Imperials to win, who are in turn fighting amongst themselves. And so, the Skywalker family, which you may remember as being the entire point of the Star Wars saga, fades into the background as we watch the Rebels continue to fight two different Empires for 20 more years. During that time it's fair to assume billions more people died and trillions more words of poorly written dialog were spoken.
"George, I don't even know what this means..."
"You say what's on the page or I'm leaving you in the fucking desert."