I don't think Halloween and Star Wars are good comparisons for Alien or Predator.
Star Wars is Star Wars. No matter what they put in the cinema for VII, people would have gone to see it.
Halloween is one of the most well known horror franchises. Horror always makes good money in the cinema, especially with the teens and young adults. Slasher maybe somewhat out of fashion, but still, you can always count on teens going into horror movies. With Jamie Lee or not. Most people, that are not fans, probably aren't following the story line between the countless sequels and the reboot anyway.
Genisys has Arnold.
It all winds up to: make a good movie. With Ripley or not. And I think thats easier without her or Newt.