Matt Thomason
Hero
IMO, it doesn't make much sense to produce a "Black Flag Rulebook" until the existing 5e one becomes hard to find. At that point, you want one just to make sure there's something for new players to keep the game going.Even if Black Flag is only an SRD (and it's not clear to me whether it'll be a full game beyond that, since a 5E-compatible SRD would allow Kobold Press to keep chugging along in perpetuity).
Unless, of course, a large number of existing players make it clear they would happily buy it anyway.