There are many viable alternatives for these researchers. For starters, all the other academic institutions! Which have their own problems, but have been increasing salaries for graduate students and post docs in response to organization (eg Penn just raised biomedical postdoc salaries in large part because of the threat of unionization, and in large part because of the competitive threat of other institutions who’ve increased salaries).
But beyond that, private sector is absolutely a viable alternative. People don’t make their choices in a vacuum, they choose the academia path because it fits their long term goal and it’s worth the short term costs.
It’s also not even remotely true that being an MD is the first choice for researchers: of the people I know in research at prestigious institutions, it is a mix of MD-PhDs and people who only ever wanted to do research. There’s absolutely not some huge population of thwarted doctors getting prestigious postdoc positions.