Despite the typical tech-bro anti-intellectual comments on this thread. As the article states. It's the money. People need to be able to support themselves.
PhDs are important because they train specialists by giving them the time and space needed to develop that expertise (something not usually available at corporate gigs). The work you do during a PhD has value, much more value than the stipend is worth. Taxpayer dollars spent on these stipends have a huge ROI because they are investing in future expertise. These PhD students are trading their cheap labor for agency over their work. However, the deal has been stagnating and stipends are not keeping up with inflation. Stipends can be low, but they need to support the students living needs.
The job market is always a bit tougher on specialists, because of that focused expertise. However, an excess of PhDs is a net benefit for society. Most won't become professors, they will filter back into the workplace and bring cutting-edge knowledge either directly to their expertise in industrial settings, or laterally to new fields.