The problem is that they say that performance has improved. But by how much? If only by 5%, then what benefit is that? It'll mean that employees are effectively getting a 25% bonus and little to show for it; and labor is typically one of the most expensive parts of an operation. By bumping the costs by an additional 25% you, as an organization, are making yourself less competitive and more difficult to survive.
Another point worth mentioning. To quote the article "employeesâ(TM) motivation and commitment to work increased because they were included in the planning of the experiment, and played a key role in designing how the four-day week would be managed so as not to negatively impact productivity." So, wait. Did productivity increase, or just not decrease? As I'm sure you know (but perhaps the author did not), they're vastly different things. Without knowing what changed it's hard to know if this is a financially viable solution. The article then goes to say "Employees designed a number of innovations and initiatives to work in a more productive and efficient manner, from automating manual processes to reducing or eliminating non-work-related internet usage". But why don't they just do this anyway and make people more efficient? (as an aside, I'd argue that internet at work can be beneficial as it let's one rest their head so to speak. But obviously too much internet is wasteful. Not sure if there have been any studies on this...)
Lastly, pure capitalist ideology simply says "if you want to survive, be profitable". How or why is irrelevant. Increasing costs to an already expensive component of an organization does little to help the much-maligned "bottom line" and help you be competitive against those with cheaper labor.
Look, I can definitely appreciate the idea of "less work can increase productivity" (after all, people who work 80+hrs are probably only marginally more productive than those that work 40, while making everybody miserable at the same time), but to assume that simply cutting off a work day will somehow magically solve our work-life balance problem -- while maintaining productivity -- is just foolish.