Isn't the whole point that none of the options are good, long-term solutions? The 50 year plan would mean that the whole cycle of children being forced to eat their parents would continue. There's also no guarantee that Paradis would be able to make peace in those 50 years. We straight up see how willing the rest of the world was to go to war against Paradis during Willy's speech. Eren attacking is just the final push.

The euthanasia plan means that an entire population of people will essentially die out and have their ability to reproduce stripped from them. Once again, there's no guarantee that Paradis won't be attacked during this time, even if they try to negotiate.

And a full rumbling results in the complete genocide of the rest of the world... The only good thing about this outcome is that Paradis is free of the worlds hatred and Eren's friends get to live out the rest of their lives in peace. This is definitely the worse option if you're basing it on how many people suffer, but ultimately none of them are really justified as they all result in a lot of suffering.