This is true, but Keynes was just encouraging a macro perspective, in that statement. An economy, by definition, can afford it's output. It's still true that

This is true, but Keynes was just encouraging a macro perspective, in that statement. An economy, by definition, can afford it's output. It's still true that what we build, destroy, deplete and such affect the future. All keyenes was trying to spell out was that we can't really save or borrow from the future outside of these kinds of things. Economic output doesn't store like that.