Honestly, the ending that we got was the best possible

A few details could have been different, but the overall story should remain the same always. Let's think: What other ending could it be?

Requiem, aka full Rumbling is the worst possible choice. Now, if you think the official ending was controversial, imagine an ending where Paradis achieves worldwide peace after a full genocide while Eren becomes the new king? Just imagine the implications of this. Eren killing his friends makes no sense and is narratively unsatisfying. It would never happen when the entire final arc theme was about stopping the Rumbling.

Another option, which would actually not be bad, would be The Rumbling being completed while Paradis still destroys itself many years later in a civil war. Now yes, this could feature the same themes of the official ending and maybe even better and more impactful. But there is a problem: an ending where everybody and everything just dies feels wrong. I know the series is quite dark, but a completely holpeless ending where all our characters die, the entire world dies and even Paradis dies latter might be overkill and just unrealistic, Isayama would never do this after 139 chapters.