What other old school game should have become as big as D&D but didn’t? Why do you think so?
I've never actually played it but probably Runequest. It had solid rules for play and a great non-traditional (non-medieval) world. Back when I read Alarums & Excursions I found the discussions about RQ engaging and fascinating. Sadly I was low on spending cash as a kid and all my purchases were for my beloved Chivalry & Sorcery. Speaking of C&S, I can't really propose it as the game which should have made it big, despite my fond memories of it. It was too fiddly and lacked "fun".