I think an key here is that the people are still interacting but in different ways. In other words even if we could change reality we couldn't necessarily change it in the way we think. In other words, a message the producers had in the interview Chora linked to last week was that the Losties decided to change everything but took no regard for just how that would affect everything they would be if the plane had landed. In my words, they were too busy trying to play God that that they never considered the actual consequences of changing the past. I think they assumed that they (or at least Jack) assumed that they would land and all go their own way. Instead, we are seeing that you can't outrun certain interactions that you are destined for, just maybe the way they take place?
It is definitely a change-up they threw in with this whole alternate universe thing.