Would you guys say marketing, or better off said the lack of, played a major role in the demise of many of the now dead mobas?
Definitely to some degree, but I would also say the commitment to supporting these games. If you're doing a ARTS/MOBA/whatever you are calling it, you have to plan to support these with updates, balance changes, meaningful new content (aka, don't just pump heroes out to say you added more heroes, they actually need to fit in the game). A lot of these games missed that part, and it's something that Dota 2, LoL, HotS, Smite, and the like have all understood and done well (with some exceptions).