>But other missions would require rare high end high performance high salary specialists who are in demand - and thus expensive. You can't rely on being able to source enough altruists that will put up with being paid half their market worth for the sake of the mission.'

That's exactly what a non-profit should be able to rely on. And not just "half their market worth", but even many times less.

Else we can just say "we can't really have non-profits, because everybody is a greedy pig who doesn't care about public benefit enough to make a sacrifice of profits - but still a perfectly livable salary" - and be done with it.