Apples to Oranges analogy. Both Terminator and Alien had compelling, memorable lead figures (Schwarzenegger, Sigourney Weaver) who were figureheads in the subsequent releases of each franchise and the key reasons for their success.
Skynet, John Connor and Sarah Connor have meaning
today after four films and a television series, but I'm not so sure you could have found many people in 1990 to tell you what any of those names meant. Likewise with Ellen Ripley or the Nostromo in 1985.
Alien was Sigourney Weaver's first role, so she wasn't a household name yet either.
Also, we're talking about original movies released in 1979 and 1984, which was a much different time than today for movies. Not as many people had VCRs and the movie rental business isn't what it would later grow to be, or anything close to today's streaming media options.
All people really needed to remember during those seven years was the general premise of either movie and whether or not they liked it to give the sequel a shot. Those who hadn't seen it -- an "entire generation coming of age" not knowing what Skynet or facehuggers were -- could at least hear buzz from those who had seen it to form an interest.
Avatar had a wider audience in theaters than either of those Cameron films and has continued to have much wider exposure than either of those movies in the years since its release.
I've seen a countless number of kids show up at my front door on Halloween dressed in Star Wars costumes. Plenty as Spiderman or Batman, a few even in Terminator wear. And the variations on Toy Story characters are endless.....
No argument from me on this one. Even if the sequels are successful, barring some changes it's going to be tough for blue body paint and loin cloths to crack the Halloween costume market. It's neither age nor season appropriate attire for children.