The concern is that there's a large enough chance that it might to be worth planning for the outcome. That chance doesn't need to be high for that to be the case. And there's good reason to believe that the size of number that has been factored up to now is not a reliable indicator that the growth rate will remain very slow.

(The analogy with the Manhattan project is apt: an adversary learning about it would have been wise to adjust their planning around the possibility of it succeeding even if they judged that it was not a given that it would)