121.5 is a dedicated frequency for dealing with actual and perceived may-days, pans and situations where a/c are lost or temporarily unsure of their position. It is staffed by highly trained RAF officers who have the equipment and knowledge to assist in these situations and the means to stop a problem turning into an emergency.
GA pilots may well misuse this service at times but I have not heard D&D staff calling for radical changes and restrictions in its everyday use as an emergency channel of communication as a result.
These days a PLOC is taken very seriously and a call on 121.5 will be made before further action is taken against a silent airliner. The guard frequency was not designed for this but now it is being suggested that the procedures for its use, real and training, are changed to accommodate the recalcitrant airliner crew not listening on their assigned frequency.
Would it not make more sense to leave what is a very important national safety asset alone and look into other frequencies, procedures or channels of communication when dealing with PLOCs?