It would be very simple for the Maryland State Police to continue asking for a good and substantial reason and to include a field in their database indicating which license seekers met that requirement. If the 4th Circuit reverses, the MSP could simply mail a letter and require those without good and substantial reason to return their licenses. I'm betting there's some sort of fine or misdemeanor which they could cite to leverage compliance.
The failure to return the license is not a real problem anyway. Practically the only time it is used is when there is a police stop or other encounter with the police. The police routinely run these and could then confiscate the license and arrest the offender for carrying without a valid license. In short, the whole argument does not hold water.