I don't disagree that there has been a mass migration of academic radiologists to the private sector. The gulf in compensation between the private and educational sector is a big persuasive force that is causing this flight from the academic realm. However, the private sector is hurting for radiologists too. Just a few years ago, as you mentioned, radiology was a field in slump -- to be even considered for a job, most residents had to go on to fellowships. These days many are skipping fellowships and heading right into the market.
While I share your concern about the shortage of academic radiologists, I believe that with the stabilization and correction of this current boom in the field, there will be a natural shift of radiologists back to the academic side as salaries in the private sector drop to a more reasonable and sustainable level. Regardless, the fear of having poor teachers shouldn't deter students who are genuinely interested in the field. As the popular quote says, "Where there's a will, there's a way." 🙂