I don't think it was filmed to end there really. I think they expected the series to be renewed. The series finale was thrown together at the last minute is my guess. The scene where they freed Pam, killed Pilcher, and then Pam and Kate have the bonding moment was all fluid and made sense. Jump to the scene where Ben wakes up in the hospital and the kids have taken over the town and there was absolutely no point to the three previous events happening.
The creatures were out of their element on an open street, they were distracted by prey and had multiple people with guns around them shooting them. Taking out quite a few of them isn't impossible. In the end of that particular scene though the humans did in fact lose.
What is epically stupid about the finale was expecting a fairly small group of kids who grew up with no computers, no internet, and pretty much no electronic devices to be able to not only penetrate the mountain but to be able to run such a technologically advanced facility. I mean, come on.