I think that's probably the most frustrating thing of all - if this had been planned as the last season, then I'm sure it would have all been written so differently!
Most of the original cast may have stayed around, and those who had already been lost (thinking Ryan and Stephen) might have been happy to return if only briefly. All the on-going storylines could have been tied up in a satisfying way, no-one would have been left up a tree, shot by their ex-wife unnecessarily, fallen badly off a cliff, etc, etc.....
I feel a new thread is needed, with the best written finish to Primeval.......