The main issue is given how the Rumbling is conducted, it would destroy Paradis too with all the ecological damage it would cause. Eren's not just killing people. He's effectively upending the entire world's ecological system, and wiping out most plant, and animal life. All the smoke and ash would block out the sun, and cause another ice age. The oceans would turn to toxic sludge. Acid rain would fall, and that along with the said new ice age would render farming all, but impossible.
He's basically bulldozing the entire world, so all the technological, medical, artistic, etc. progress the places outside the Walls made would be lost, too. It would also render the goal of the series moot too because there's nothing to see outside the Walls once he destroyed it all. Yeah, they killed all the titans, and outside threats, and shit, but what's the point if all that's left is a smoldering, barren wasteland that's eventually gonna kill you too anyways?
Lastly, a full-scale Rumbling was not needed as small ones used to take out other countries military capabilities could've worked just as well. At least long enough for negotiations to be possible. True, it might not have worked given how dumb this world is, but it still should've been tried first.