If you'll drop the "in an efficient utilitarian way" clause, then I submit that quite a few working parents qualify as an example of both "career substantially less enjoyable than what they would otherwise have been doing" and "successfully stuck to it". Choosing between the more-enjoyable (artistic, non-profit, low-stress, whatever your preference is) career and the more-likely-to-put-your-kids-through-college career is practically a stereotype.

If you'll go one step further and allow "themselves" to count as "people", then I'd say that nearly every person in history qualifies as an example of "career substantially less enjoyable than what they would otherwise have been doing in order to help people". Unless you have very extraordinary preferences, skills, and/or luck, odds are that the activities you enjoy most are relatively unproductive activities that other people also enjoy, and that this weak demand-to-supply ratio prevents those activities from being paid a liveable wage. An Office Space quote keeps running through my head: "If everyone listened to her we wouldn't have any janitors, because nobody would clean shit up if they had a million dollars."