It's actually been mentioned several times.
The problem is that the MD schools that open, because they need to be LCME accredited, need to provide their students with strong, teaching-hospital rotation sites with resident teams. DO schools have no such obligation, resulting in poorer quality of clinical education.
The number of residencies also is remaining constant.
The end result is that there are far more students competing for desirable residency positions... and many of these applicants are from the new MD schools, who, because of their superior clinical education, will beat out the DOs for desirable specialty spots.
So the fear is that more DOs than ever will be "funneled" into primary care, seriously limiting carreer options, and cementing the DO degree as a "primary care degree" and pigeonholing DO students. There are also many who fear that the proliferation of "subpar" DO schools will seriously hurt the reputation of the DO degree.