That's fucking stupid, and is basically just an excuse to remove ALL other options from the table.
"Unless this is 100% guaranteed to work, you HAVE to murder EVERYONE else instead." That's a completely unreasonable and nonsensical restriction to put into place, and I'm not even going to entertain listing hypotheticals with that kind of bullshit handcuffing in place.
Basically, if it had been up to you, we would ALL be fucking dead from nuclear war by now. Because your entire justification is "Well, if there's even the slightest chance they might annihilate us, then it's too dangerous NOT to annihilate them! Don't explore any other options. The risk it too high."
I think my biggest problem with the writing was the way Willy Tibur was able to unite the entire rest of the world against the Eldians with a single speech. Even though Marley is the ones that had been conquering them and dropping titans on all of them for centuries. Even though that was all Marley's doing, Tibur was somehow able to convince them that, "no, the people we keep imprisoned and basically use as missiles against you are the problem, and you should all help us to go their island and get us even MORE of the power we've been using against all of you".
And they all just agreed to that.
Like, realistically, Eren should just be able to smash Marley's military and government. Free his fellow Eldians, and then begin negotiations with the other countries, having just freed them all from Marley's oppression. Hell, the Japanese-ish country was ALREADY willing to ally with them.
But no. Everyone has to die instead. Every innocent man, woman and child on the planet that's not on the island with him. Fucking stupid.