If they are predictable then many people have done an extraordinarily bad job at projections. Because there are a lot of real howlers out there then if you look at proven wrong population projections of the past both from Malthusian demagogues and official sources trying to do extrapolations.
Although it is true we can only "add" at 0 for new humans if you want workforce ready new humans immigration from elsewhere is required.
Well Malthusians are frankly crackpots trying to make a political point which the numbers (especially now) don't support. So that they come with numbers that are found lacking ... is not a fair attack against serious people.
Which specific forecast did you think was so bad ?
Although it is true we can only "add" at 0 for new humans if you want workforce ready new humans immigration from elsewhere is required.