This may be good advice when the socially-beneficial job in question is something less expensive than medicine. But doctors are expensive. Most of us can't afford to pay for someone else to be a doctor.

But, still, let's consider someone whose skills and interests do make it feasible for them to do so. Let's say they could work as a doctor (earning, let's say, $150k/year, and perhaps costing $200k/year "fully loaded" -- for the avoidance of doubt, all numbers here are completely made up) or as some kind of financial analyst (earning, let's say, $350k/year). After tax, perhaps our hypothetical financier is getting $240k/year, which means they could pay for a doctor and have $50k/year left over. Alternatively, they could be a doctor and take home ~$110k/year.

I repeat that all these numbers are made up (except that I checked the rough relationship between pretax and posttax income). But the overall point is pretty clear: the person we considered could pay for someone else to be a doctor, but only by taking a much better-paid job and ending up paid much less. They'd have to enjoy finance a lot more than medicine for this to be a good trade.

On the other hand, they probably can do more good by picking some job that suits them and pays well, and giving (say) 20% of their income to an effective charity. But it probably won't be (either directly or indirectly) by paying other people to be doctors.

Never forget the power of marginal contribution. Instead of becoming a doctor you can help several people at the edge of becoming a doctor. Even if this happens only once you are already more successful than by becoming a doctor yourself.

Of course, if a lot of people are already doing this. you need to find something else to do.