Honestly, instead of annihilating the humanity outside the walls, he could have just wished that their history of Titans be erased from everyone's mind, book, everything and make the titans disappear. Let the people on the island be treated as normal people and live a normal life.
I think, that you can't wish that
1. Erasing titans means changing history and erasing eldians
Eren and the others wouldn't exist
2. If it was that easy ymir would have done it herself, because i don't think, that she was happy being married to that pig
3. Paradise island has still valuable ressources, which means-> being attacked by other nations
I hope I'm not the only one who still has a little hope in Eren and hopes he is smart enough to be doing this on purpose. It makes sense... faced with a threat of genocide, humanity has no choice but to unite.