Most "college should be free" proposals fail under scrutiny because degrees tend to fall into a spectrum of utility:
On one end are degrees that impart skills that are needed and desired by the economy. By and large, graduates of these degrees _already_ enjoy sustainable wages, because the market recognizes and rewards those skills. Government subsidy for these degrees tend to have marginal benefits, because it is already a sound financial choice for students.
On the other extreme are degrees that, like GP identified, were never meant to be economically sound. They focus on intellectual ideas and curiosities, rather than skills actively needed by the economy. Government subsidy here would be little more than subsidizing hobbies and other activity that do not provide economic returns.
There _are_ specific degrees where there's a public policy interest in encouraging the study of topics not adequately compensated by market forces. Most of these are already covered by specific subsidies, such as scientific research grants.