"If this works the way I planned, there will never be an Apocalypse to stop."
And Dean actually considers it. If Henry goes back, John would be raised as a Man of Letters, and so would he and Sam. With that knowledge, Dean might be able to get to Sam before Jake kills him at Cold Oak. He might know a different way to bring his brother back and avoid selling his soul, avoid going to Hell and breaking the first seal. Sam wouldn't be left alone and desperate, with no one to turn to but a traitorous demon. Sam wouldn't get addicted to the blood, and he wouldn't release Lucifer from his cage. Sam wouldn't be so wracked with guilt that he would dive head first into nearly two centuries of constant torture. Cas wouldn't go crazy with power, destroy Sam's wall, and unleash the Leviathans. Bobby would still be alive. Ellen, Jo, Ash, Rufus, Pastor Jim, Caleb, hell, everyone would still be alive. They could all be one big, happy, sitcom-worthy family.
But then he's thrown back to a memory from a future that never happened. A future where Sam lost the battle to Lucifer, and Dean, Cas, Chuck, and others battled Croatoan-infected populations and angels and demons to a futile end. A future where Dean's neck was crushed by his brother's own heel. A future the Devil himself told Dean was unavoidable.
"No matter what choices you make, no matter what details you alter, we will always end up here."
And Dean knows he can't let Henry go, because there will always be an Apocalypse to stop, and changing any one thing means risking that a whole number of things can go from bad to worse. And even though Dean will always wish things had been better, he will never, ever risk them being worse.
Before Dean can say anything to convince Henry not to go, his phone rings.
On the way to the coordinates Sam got from Larry, Sam has fallen asleep in the passenger's seat. Dean is still thinking about the Devil's words. "Here" was just barely a year away, and he wasn't sure what it was, but he knew something was coming. Something very big and very bad was coming.
AN: Can we just acknowledge that Lucifer talks about fixed points in time better than the Doctor? I get shivers every time I think of that line!