Okay, this is true based on the examples I can think of, although there aren't that many. The Universal monster sequels of the '30s and '40s are really fast and loose with it, watch the James Rolfe reviews of the Frankenstein movies and he points out a bunch. Alghough only the first two are ambitious movies, after that they get really schlockey.
You don't have alot of big movies getting sequels that continue the story the way Aliens does until after The Godfather movies, which is just before the VCR era.
Aliens is right around the point where about 50% of American homes had a VCR, if I remember correctly.