If you both have to cover a cost of living, and care about improving the world, there is some most efficient strategy which allows you maximise the latter, given the former as a constraint. (This obviously varies by individual, depending on your abilities, available work, etc.) How can one be sure that working at an 'altruistic' job is optimal, as opposed to for example working at a very highly paid job in some pointless but not harmful field, and contributing either some of your money, or some of your increased spare time?
The example of someone who doesn't care at all about altruism and who has maximised wealth while causing significant harm does not establish the right strategy for people who do care somewhat about both things.
It is not hard to decide if you're hungry or if more money is better than less money.
Moreover, many businesses operate under the "everyone is replaceable" model, implicitly adding "without much effort", but that's not really true. There are plenty of examples of political movements that never recovered from their leader or figurehead leaving. You can't just take any other politician and swap them, their positions are too personal. That's an extreme example, but I think it applies to most job positions that aren't just about following checklists, in varying degrees.