Well for that subset, I would agree with you. I mean come on, you are making oss stuff that provides some value for some people, and you are struggling to get a decent income from other channels? It would make sense to try and maximize the surface area of all potential income generation activities.

But that's just a small subset of all cases, as you mention.

I've lived the opposite case: consultancy was the idea, but not enough people came in, with not enough frequency.

Talking generally, it feels ironic that if difficult to understand things get polished and ironed out, that source of revenue might and probably will dry out. So an incentive would exist to keep consultancy needs existing. (Edit: I am just digressing on this last thought, not talking about my particular personal experience)