The question is sort of valid though. For instance, I've heard cardio thoracic surgeons coming straight out of residency have trouble finding jobs because there just isn't a huge need. There are quite a few threads questioning the future of Anesthesiology, Radiology, and family medicine due to the encroachment of mid level providers or outsourcing.
If a field was made obsolete, I doubt it would happen overnight. Physicians that are currently in the field will probably (wisely) keep their jobs rather than try to find the same position elsewhere, there will be less open jobs for new residents out of residency, but probably fewer residents completing that specific residency due to its instability. A few individuals will have a hard time finding employment in that field but can probably find something else to do.
this is not exactly correct. cardiothoracic surgery isn't making millions like in the past, but they are still the highest paid surgeons out there, out of any doctor. they saw the biggest hit in salaries however thanks to interventional cardiology.
the natural dynamics of any profession is that when it becomes obsolete, people dont go into it. last year there were two spots for every applicant in cardiothoracic fellowships. the numbers are now largely equallized, and all this new talk of CABG vs. stent will likely see a resurgence, albeit small, in their business/income, though most likely not as high as before.
the thing is, pediatric CT surgery will never go away; coarctations, valve replacements, etc. will not go away for a while, and can't really be treated medically, so the CT will be needed.
an interesting historical tidbit - when fleming discovered penicillin, infectious disease physicians committed suicide in record numbers fearing their entire life's work is now lost with no future prospects.
looking around today, we still require ID specialists, as we will with most other specialists out there.
a more realistic situation is that your field is no longer in high demand, but your job will most likely never disappear. after all, history has shown us that no one field will reign supreme, and a new field (generally a surgery) becomes the new high paying king. but it doesnt happen overnight.
one good point is that doctors do protect their own fields. an example is neurosurgery; neurosurgical residencies train their residents in endovascular neuroradiological techniques. this protects them from losing all their vascular patients to neuroradiologists and starving off the neurosurgeons. but what about interventional neuroradiologists? well for now it seems that they are both in such few supply that wherever you go you wont be starving for patients. but doctors aren't stupid, and will always ensure that they will have work. and not just work, but 6-figure salary work, so it shouldnt be a HUGE concern to you.
