Why do people make arguments like this?

"Work" isn't a finite thing. It's not like all the people in your office today had to complete 100% of their tasks, and all of them did.

"Work" is not a static thing. At least not in positions of many knowledge-worker careers.

The idea of a single day's unit of "work" being 100%, is really sophomoric.

Also, If 100% of a labor force now has 80% more time...wouldn't it behoove the company to employ the existing workforce in more of the revenue generating activities? Or find a way to retain as much of the institutional knowledge?

Doom, fear-mongering and hopelessness is not a sustainable approach.