Like you point out in your second paragraph, some percentage of employees are automatically going to start looking for a new job and treat their 80% salary as a severance package. Being in a competitive job market, this is going to be pretty much every above-average employee at your company. With layoffs the company gets to choose who gets impacted, and the process is immediate rather than drawn-out.
The reality is that too many VC funded companies hire too eagerly and can operate perfectly fine (in some cases better) with a smaller, more focused workforce.
There is no coming back from a major across-the-board paycut, which is why no company chooses that option.
BTW your question is answered in the last paragraph of the article.