Who knows how long they actually have until the big bad is summoned, the girl is killed, the treasure is spent, or the dragon summons help. If they go away and leave a situation uncontrolled, they can be fairly sure that things will likely have changed while they are gone as the other side continues to act. Or at least that is always the assumption on their part, even when I, the DM, am going to change up very little.
I guess it boils down to a play style where, while my game may revolve around the PCs, the world never does.
In the last game of 4E that I ran, quick run down of the premise of the adventure was that a very special girl was born into the world 10 years earlier. She held a very small fraction of power (the game setting was FR) which had been stolen by a lycanthrope when he stopped a ritual which would have allowed an asperct of Malar into the Realms. The power had passed from the lycanthope to his daughter when she was born into the world.
The faction of lycans who wanted the ritual complete, attacked the family on the day of the daughter's birth (if anyone is interested in my prologue you can read it here on my campaign site:
Dark Side of the Moon - Epic Words) but the attempt failed. Ten years later, the moon would be right and the chance for allowing the ritual to take place was once again possible. This is where the PCs enter the adventure. The PCs witness the kidnapping of the girl and find out why from the people that hold her secret.
The people, a cloister of monks, who told them about the girl's secret also told them that the ritual would take place in three nights time, on the night of the Feast of the Moon.
One of the younger players asked what would happen if they didn't get to her in time. I let the player and the rest of them know OOC that my game world continues living, even when they take side ventures or want to pursue other venues of interest.
If they chose to follow up and attempt a rescue on the girl, they could possibly stop the ritual and avoid Malar's aspect from entering the Realms. If they chose not to attempt the rescue or they failed in the rescue attempt, then the aspect would enter the world and it's power would be used by the lycan faction to help destroy the city of Baldur's Gate; the Realms in the campaign would be changed for ever on.
You could see the look of fascination on their faces, for some reason they found this really interesting and cool. I assume playing in previous games under other DMs, they had not experienced a world that changed around them because of or despite what they chose to do within the campaign world.
And that's the way I always run my game sessions. Players have complete freedom, no scripted adventures or pre-written story lines. I toss out small hooks of things going on within the campaign world and they can choose which ones to follow or ignore. The world keeps on turning, regardless.