All other productivity increases from back-office type jobs have historically been eaten up by expanding bureaucratic requirements. These jobs tend to not get replaced by technological improvements, but rather the requirements demanded of them expand to meet the product of existing headcounts and improved productivity.
In this case, the likely result is a wash as increased ability to respond to administrative requests gets met with increased ability to make administrative requests.
AI is fundamentally different than a new version of excel and will likely be able to scale to any dumb bureaucratic requirements dreamt up.
There are less administrative assistants now that productivity tools are better and executives more competent (e.g. 25 years ago I worked at a company where the executive recorded his memos for his assistant to type).
Most of the memos that I receive from "above" are edited and formatted by an assistant. These are also the ones that nobody reads. I use a primitive form of AI to process them. ;-)
In the future, instead of receiving half a dozen of those memos per day, we could receive hundreds or even thousands.
In this case, the likely result is a wash as increased ability to respond to administrative requests gets met with increased ability to make administrative requests.