Even if AI can't replace an entire worker, they may still be able to help one worker do the work of two which by itself could lead to massive unemployment (at least in the short term.)
In this instance, it may not lead to massive unemployment as it could be simply that we do more white collar work than we currently do. White collar work, for the large part, is not born out of necessity, as white-collars, we do not produce things, we oversee the process, we market, we sell, we do accounting, we support, and so on. The more of these we do, the better our companies can compete. But there is no inherent reason that a mature company could not reduce its white-collar jobs to a halt if there was not competition. White-collars work because the competition also hires and gets them to work. Hence we say "productivity is increasing but we have to work more". So even if AI leads to massive improvements in white-collar work productivity, as long as it is not entirely replacing people, it may not lead to job losses at all.