If we wait until there is no topsoil left, even more billions will die, and it won't be in a controlled and purposeful way like it would be if we do it now. Hell, maybe even no one has to die from starvation. Maybe just limiting births would do the trick. Or any other number of ideas that don't involve mass murder or mass starvation or going back to 200 year old farming methods.
People were advocating for much the same thing in the '70s when Erlich's The Population Bomb came out, and thank God we ignored them. And yet a single hectare of land produces multiples of what it produced then, allowing enough food for everyone (though it doesn't always get to the people who need it, admittedly).
If I had to bet I'd say in 100 years we'll be producing even more food per person, so I consider any plan that would have people starving forced to limit reproduction to be off the table.
So it's idealist to want a sustainable society that doesn't destroy the planet a short couple hundred years after discovering fossil fuels and industrial food production? How is that idealist? Idealism is thinking that the current situation is somehow salvageable without extreme overhauls.
And plus, no one is saying to go back to oxen and carts. That is fairly ridiculous so my response was equally ridiculous.