I think the point would be a <$1k sub generically designed isn't as attractive to someone shelling out $6k, $12k, $25k for the upper end of Heritage speakers. I'm talking about a sub designed to compliment the Heritage line specifically. Certainly there was no sub in 1957. Some of the tech in the Heritage line wasn't around in 1957 as well. Corvettes didn't have anti-lock brakes in the 50's either.
I think it's time for Klipsch to do it. The bass rolloff on so many models leaves much to be desired, and for every Heritage lover that says I have tons of bass coming out of my LaScala, Cornwall, etc., there are plenty others who use quality subs with them that are not in Klipsch's other lines.