By now I am resigned to the expectation that it will never happen. The question-and-answer engine that powers SE should be good enough to attract communities of experts around any topic. Good enough so that the ease of use in getting answers (experts have questions, too!) should outweigh, as a plus, the minus of "donating free labor". To make this happen, SE would have to partner with one of several major academic institution or professional association. That, however, would be a stretch too far for the good people at SE, who are software developers and comfortable only in their own environment.
History.SE will either languish and wither on the vine or continue as an amateur community for history re-enactors, aspiring writers of historical fiction, crossword puzzle solvers looking for a change in pace, et cetera. That is not the worst thing that could happen. At one time, this Stackexchange looked in danger of being infiltrated by holocaust revisionists trying a soft-shoe approach, asking "harmless and innocent questions". It was one of the reasons why I joined, to put a stop to this.
Keep in mind that not every amateur history enthusiast is a friendly hobbyist. Many are conspiracists, racists, whackos, creeps. Keeping them out is a thankless and never-ending task but it needs to be done.
Not to mention an easy way for people interested only in Stackoverflow to quickly reach 101 rep, collect the "association bonus", and bypass the new-user restrictions on SO.