Ronson2k3-I follow you on your line of thinking. But if you'll remember from Alien, the ship was derelict, it crashed on LV426. Due possibly to one of the aliens getting out and hosting in the pilot of the ship. Would that not suggest that the Pilot and his race were on their way to drop off the cargo somewhere? Then oops, so sorry Mr. Pilot, you're dead. You are correct with the parasitic element, but that just goes to show thy were designed specifically as a bio-weapon already. (At least to me)
You are correct with thir entire survival depending on a living host, but not the entire species. Just the ones on LV426. What would it benefit the Pilot and his race, of only building one cocoon/cargo ship? There had to be many more ships like it in the universe.
To me, the reason the aliens never tried to get off LV426 was because they weren't designed as pilots. While they are smart and adapt quickly that just wasn't "programmed" into them. Kinda like a bee hive or ant colony, each ant/bee has a specific job to do.
Now if the Humans had never come along, nothing would have happened. While the ship had been there on LV426 for an undefined length of time, those eggs would have more than likely died off over time. But other ships would have dropped off their deadly cargo and spread across the universe, quite possibly reaching earth at some point in the distant future.
Just my 2 cents worth here.:mrgreen: