IFR lost comms regulations in 91.185 are written for the worst case scenario of pilot and ATC operating environments. We do have areas in the world and the US that do not have radar coverage (such as mountainous areas) where ATC literally cannot "see" you on radar, therefore the only thing they can do is clear traffic from the route, altitude, and destination they are expecting you to go to. When we recieve a clearance limit, we are required to continue to it, unless we encounter VMC and can land elswhere using that provision in 91.185. We cannot just change where we are going willy nilly because of "PIC emergency authority"...the FAA has given us the exact way to comply in the regulation. Lost comms is not considered to be an emergency that allows for deviation from the plan; fire on board the aircraft or electrical failure, however, would be full emergencies and allow for deviation. CFIs, if your students do not know the importance of ETA and clearance limit when it comes to lost comms, they NEED TO.