There's no justification for changing the modal verb.In a question, I can see the following, but it'd better be interpreted as "Everyone could benefit from that." as it's an opinion in a rhetorical question. "Couldn't" seems to mean a low probability. So "who can't benefit from that?" can be converted to "Everyone can benefit from that". What do you think?
Q. Who couldn't benefit from that? => Everyone could benefit from that.
ex)...For one thing, volunteer work raises one's confidence. When you help someone else, it makes you feel better about yourself. And who couldn't benefit from that? Students will realize that they have the power to make people's lives better...
Student or Learner