Starting "fresh" with Terminator and killing everything post-T2 Halloween-style makes sense. Everything after that second film, at least in my opinion, has been dire and utterly forgettable, with nothing of note to them to make me want to revisit them, or even remember that they exist. And for those out there that do care about them about about the continuity post-T2, it is a franchise that involves time travel so if you really wanted to you could chalk it up to being an alternate timeline.
With Alien... even the weaker films are at least conceptually interesting and are executed in such a way that they feel as though they actually have merit for existing, that they have something to say and contribute to the universe and, even more importantly, as films in their own right.
Nothing against Blomkamp, I do enjoy the guy's stuff for what it is, but his take on Alien is, at least in my humble opinion, not at all what the franchise needs. Let him do Robocop, which is much more in line with his grounded, hyperrealistic sci-fi visuals and tongue-in-cheek satire. And let Alien wrap up the story that it is currently telling, and then move forward into some new uncharted territory.