They really could've made this into a long-running franchise if they'd wanted to. The Krites aren't necessarily tied down to the Brown family like Gizmo is to Billy in
Gremlins
, or the sharks are to the Brody family in
Jaws
. They're aliens, plain and simple, and could come from plenty of places in the universe (other than the Saturn Quadrant) and show up anywhere on Earth. You could have 8 movies all telling a different story if you wanted.
As for the actual topic, I don't think C3 was pointless. It was a fun little flick. It wasn't the quality of C2, but as its own movie it's just fine. C4 is where the series really drops the ball.
Cena doesn't overcome any odds. He's a wrestler with a God Mode cheat on.

All the critter movies were pointless except part 2

Thanks for agreeing with me. 3 did have a kiddish feeling and yes 4 was boring. This is how i rank them: 2,1,3,4

The first two movies are great and the second one wrapped things up nicely - the third and fourth movies were kinda pointless, 3 is ok but it's more kiddy-ish and it doesn't feel right without the small town setting, 4 is pretty boring