The Big 4 hire thousands of new college grads every year. New hires start out as Staff, then if they work out they get promoted to Senior, then Manager, then Senior Manager, etc. Very high turnover (to be expected, many college grads haven't matured). As it stands right now AI can replace most of the staff-level jobs. Big 4's problem, though, is that if they eliminate or reduce staff hires they won't have the people to work up the ranks to the more senior positions (which as of now are not threatened by AI).
But the point of the article is a great one to consider: What happens to all those college grads if the jobs with accounting firms dry up?