"There is also the question of where the relevant expertise is most concentrated, and re that there cannot be any doubt that SO wins by a mile."
This is a ridiculous claim. C experts are a very small sliver of the StackOverflow community. Many of the answerers on SO are Java experts or C# experts or Haskell experts or whatever. C experts are maybe 5-10% of StackOverflow [source: PIDOOMA].
On EE.SE, though, probably half or more of our answerers are regular C users. If anything, C expertise is more concentrated on EE than SO.
Of course, you may reply,
Okay, but 5% of StackOverflow is still bigger than the whole EE.SE community
If our policy was, whenever there's a bigger community available to answer a question, we should refuse it, then StackOverflow and EE.SE would never have come to be.
When SO was first started, they would have had to respond to every C question with, "we refuse to answer this because there's more experts on
comp.lang.c than here."
And when EE.SE (or Chiphacker or whatever we were at the time) was started, we would have had to say, "go ask this on AllAboutCircuits. They have many more people to answer your question there than we have here."
So just because there's a bigger community available elsewhere (SO) is no reason we on EE.SE should refuse to answer a C question.
Regarding the comment, "It's the absolute numbers that count."
My point is that it is not. If we want to grow our community, we can't do it by refusing to answer questions until our community is bigger.
If you think a question could get better answers on SO, it's perfectly reasonable to leave a comment that OP should try their question on SO. I've often done this for questions that overlap with physics.se or dsp.se.
But if you say it is "off topic" for EE, that's going further. That's not just giving the OP advice on other places to ask a question, it's saying that EE.SE should refuse to answer the question, like we refuse to answer questions about consumer electronics or the airspeed of an unladen swallow. It's saying that the EE.SE community should restrict itself from answering this type of question.