[AlexandrB]

To paraphrase: "This is going to happen a long time from now, so it's not my problem and I'm sure humanity will have fundamentally changed by then."

This is outright utopianism. Look around you. The backlash to even modest immigration has become increasingly violent and reactionary (see: Brexit, Trump). Do you honestly think that at some point everyone will be OK with just opening their borders to billions of people? And how many do you think will die trying to make that journey?

[Mikeb85]

200 years ago slavery was still rampant, war was everywhere, and the vast majority of people were serfs/peasants under the yoke of various tyrants/monarchs. We didn't have electricity, motorised vehicles, etc...

Given everything that's happened since, forgive me for being optimistic about our future.