Even if we conveniently assume that all the people of the future smaller population have it in them to do the jobs AI is not yet good enough at (and none of them are would-be copywriters because that's where their skill ceiling is), surely the game-theoretical shift from "if you, aspiring original writer, don't make it for some reason or another, you'll have to settle for a lower-prestige/-pay writing job" to "...you'll actually be stuck with none of the skills you acquired being of particular value to anyone". Writing as a career would assume a risk profile more akin to trying to become a K-pop idol.