Honestly, the rate limiting factor here is funding. For my lab's last grant application cycle, paylines were below 4% (meaning that the top 4% of applicants got money and the rest were SOL). Sure, you can apply again, but you need new ideas or data if you want to submit more than twice, and there are only so many cycles before your existing grants run out.
The point is that most people with back-up plans aren't taking risks like this, and I doubt MD-only scientists are the answer. If MD/PhDs struggle with funding, someone with $300k in medical school debt would struggle even more, especially when you consider that they have to hunt out their research training during residency and many fail to secure grants that will allow them to continue in research.
The answer is better funding. This country needs to support research in a way that offers some sort of job security to scientists. If not, the physician scientist path is doomed, at least for those of us in basic science research.