Yes, if everyone becomes a hedge fund manager and no one a doctor, then there will be no one to accept all the wonderful donations.
(except I guess nurses, who are totally people who exist and who I secretly suspect would just keep treating people perfectly well if all the doctors suddenly disappeared).
On the other hand, if everyone became a doctor, then we would all starve to death because there are no farmers to grow food.
Since we are allowed to give the advice “become a doctor” without worrying about everyone starving, we are allowed to give the advice “become a hedge fund manager” without worrying about a total collapse of medical care.
(I guess the short version is that becoming a hedge fund manager is the right decision on the margin, and until effective altruism is WAY bigger than it is right now, the margin is the correct way to look at this)
(also, some effective altruist organizations are moving away from earning to give in favor of becoming things like economists doing development aid for the World Bank, but I think the instructive point between “doctor” and “manager” still holds).
Obviously this is just for the average person. If you are Kanye West, the appropriate thing to do is become a rapper and make millions of dollars that way. Once again, all you can do is transmit statistical facts and tell people to adjust for their own situation.