Sustainability relies on _future_ tech. If we need to know if we can feed 20% more people in 10 years we need to know if we can produce 20% more food in ten years, using what technology is available then. Not what is available today.
And we (and this is what people who keep saying we can't support all those people never take into account) _can count on the extra minds of those 20% to do their uttermost to solve the situation_.
Meanwhile, farming and food production is one of the least profitable industries to be in... Trust me, when the need comes, there will be a way.
Obviously, the situation is different in poor countries - but just take a look at all the expired food we dump from supermarkets, restaurants, offices and homes - it sure is more than enough to feed a lot of those who are starving... if only we had teleporters...
future tech is not taken into consideration when defining something as currently sustainable. It currently is or it is not sustainable, not "it is sustainable due to inevitable future tech developments".
And we (and this is what people who keep saying we can't support all those people never take into account) _can count on the extra minds of those 20% to do their uttermost to solve the situation_.
That is what people never take into account.