After a career in HR, I will say that two part-timers instead of one full-timer is less efficient, and adds expenses for the company in many ways. It’s not just an equal replacement. For example, now there are twice as many employees, so there are space problems, logistics problems, like parking, all processing (like payroll) takes longer because there are twice a many to handle, and all the little issues related to employees are doubled. More hiring time. More turnover. Many other things like that. So being forced to run a company with more lower-hours employees has a negative impact on the productivity of the company. If the company is on the edge financially anyway, it might not survive such a change.
All true, but in certain industries - like the restaurant business - those factors have far less impact and you’ll certainly see a much greater proportion of part-time workers.