I've read an interview with Gregory Lewis on 80000 hours. He argued that due to counterfactual, doctors don't make a big difference because doctors are already highly competitive, so you don't make big impact especially if you work in rich countries. There's a problem: You can still make a difference by being a better and more patient doctor. I don't know the doctors in America, but in East Asia, not every doctors are good. Some just want to make money and they treat the patient poorly, making them suffer more.(such as misdiagnosis) So, if you can be a good doctor, the counterfactual case would be" You replace a worse doctor than you". I don't know how valuable would it be, but this shows doctors in rich countries may be more altruistic than normal careers. Also, you need to consider how prolonging one's lifespan influences his/her relatives' suffering. Some people feel desperate if their relatives pass away. Being a biology researcher may be more valuable than just being a clinical doctor in the long run, but, I think we may underestimate a doctor's impact.