By how the world has been painted and told by Isayama, there just can't be a reasonable way of writing an ending where the marleyans would stop their hate and oppression of eldians. These characters are just chasing a pipe dream, and haven't even had a serious discussion what happens if they can stop the rumbling midway... I'm already half-disappointed thinking that probably they will succeed stopping Eren, since Isayama is probably going for some half-thought ending of world in peace where by deus ex plot point, everyone are just friends..