I was sad enough - for another forum to draw up how the writers intend for it to work:
There are at least two ways of interpreting time travel:
1) There is only one timeline, when you go back in time and change things everything from that point forward is changed. e.g. Go back and kill Hitler to stop WW2
2) For every single possibility an alternate universe branches off. e.g. One day Hitler is eating his toast, he chokes and dies, WW2 never occurs. In another he finishes eating his toast walks outside and is run over and killed, WW2 never occurs.
The main issue is always going to be that Trek has played time travel in both ways. The mirror (alternate) universe that was triggered by Cochran storming the Vulcan ship in First Contact and enslaving the Vulcan race. But also altering a single timeline - as pointed out - in First Contact the film where Picard goes back to stop the Borg.

There are at least two ways of interpreting time travel:
1) There is only one timeline, when you go back in time and change things everything from that point forward is changed. e.g. Go back and kill Hitler to stop WW2
2) For every single possibility an alternate universe branches off. e.g. One day Hitler is eating his toast, he chokes and dies, WW2 never occurs. In another he finishes eating his toast walks outside and is run over and killed, WW2 never occurs.
The main issue is always going to be that Trek has played time travel in both ways. The mirror (alternate) universe that was triggered by Cochran storming the Vulcan ship in First Contact and enslaving the Vulcan race. But also altering a single timeline - as pointed out - in First Contact the film where Picard goes back to stop the Borg.