One problem with that is if PhD programs start doing that across the board, most academic departments, especially those not ranked within the top 50 would need to severely curtail or even shut down their PhD programs altogether.
Something most are loath to do as having a PhD program is not only perceived to lend more prestige to a given department and university, but also provide a steady source of “cheap labor” in the form of grad students serving as TAs, RAs, and quasi-adjuncts*.
- Advanced PhD students who are full-instructors of college courses in their own right....sometimes with their own TAs as was the case in some friends' courses at a few large private research I universities in the Boston area.