yes, there are periodic oversupply and undersupply of various medical specialties (just like oil, labor, money, real estate agents), and medical students do react and choose accordingly. ER was hot for a while, and radiology was not so hot for a few years after 2008. Now surprisingly radiology has held up despite AI fears, because of baby boomers starting to get cancer, and ER is down, because private equity went nuts investing in urgent care centers before covid.