Check out the graphs presented, particularly the last one. Default is very likely for those most likely to have student loan debt (younger workers in lower quality service jobs), but there isn't a systemic risk as the federal government is the insurer of last resort. These are loans in name only; they are, in actuality, taxes you can't default on (or so was traditionally thought; we're making headway in having the ability to default on these obligations and get out from underneath them [1]).

[1] https://www.natlawreview.com/article/student-loan-discharged...