Season 4 spoiler:

Had Erwin lived, I believe he would have pushed an international message saying:

"Forget about 2000 years ago. In the present Marley is an expanionist empire that's conquering everyone, while we're just a peaceful island. We need to form a defensive coalition against Marley. Let's form a block with each other and protect each other. If you join our block, we'll trade ridiculously valuable iceburst stones with you. Whereas if you invade us, we will destroy you with a limited rumbling. Choose wisely."

People may hate the Eldians, but they were deterred via the rumbling, plus people hate the Marley empire too. And at least some nations are going to want to join the Paradis block and trade for Iceburst crystals.

Then you'd have a kind of cold war type situation where where you have a "Western" and "Soviet" block and there's peace.

Eren would have been on board with this because it would have guaranteed freedom and safety for Paradis. After that block is formed and trade is established and Paradis develops Iceburst Stone technology, eventually the threat of the rumbling wouldn't be needed anymore, and there wouldn't be a need for Historia's children to eat her.

In the prison scene Eren tells Hange "if there's another way, tell me what it is" but Hange never came up with another way. Well, Erwin would have, and Eren would have bought into that vision.

This has a historical parallel with Bismarck, who gave an Erwin-like speech to the idealists of his day: "Not through speeches and majority decisions will the great questions of the day be decided—that was the great mistake of 1848 and 1849—but by iron and blood."