Granted, such a person might think they could make such a large contribution to the field of moral philosophy that it would be comparable in impact to other research fields. This seems unlikely.

Unlikely that they would make such contribution? Yes. Unlikely that they think they would make such contribution? Maybe no.

But I guess they probably don't even think this way, i.e. don't try to maximize their impact. More likely it is something like: "My contribution to society exceeds my salary, so I am a net benefit to the society". Which is actually possible. Yeah, some people, especially the effective altruists, would consider such thinking an evidence against their competence as a moral philosopher.