In the end, it's each to their own. Certainly, people do end up falling by the wayside or being disenchanted by the profession. The same happens in other professions, but perhaps the feeling is greater with medicine due to higher expectations placed on its ability to deliver job satisfaction?
I doubt a high ATAR correlates with successful entry into competitive specialties, or even post-graduate performance.
By the way, did you end up going corporate?
GP isn't really the fall-back specialisation it used to be. You still need to go through various exams and a selection process, which is becoming less forgiving. Many people choose it for a number of reasons other than as a fall-back: variety, flexibility/work-life balance, patient contact, business-minded. When people change specialties it usually is a case of changing priorities. For this reason, some people also leave medicine altogether.