Here's the entire sequence of events that takes place in Supernatural season 15's flashforward future timeline. Returning from its midseason break, Supernatural's "The Trap" sees Dean and Castiel venture into Purgatory to complete a spell that might help bring about the downfall of God. Meanwhile, Sam and Eileen were captured by Chuck (props for the "come on, Eileen" line) and in order to drain the younger Winchester of all hope, the omnipotent villain showed Sam what the future would look like if he and Dean actually won.
Of course, Chuck has been manipulating the Winchesters from the very beginning, arranging their entire lives around his own desire to be entertained and tell a good story. There's no real reason for viewers or Sam and Dean to trust God's words but that issue is addressed in the episode itself, when Sam voices the same concern. Ultimately, Sam comes to believe that the visions are an accurate depiction of his future timeline, and the audience are invited to accept the same reasoning. Here's how Supernatural's future would pan out if God was defeated:
April 17th 2020 - God has been contained thanks to the spell provided by Michael. Sam, Dean and Eileen remain living in the bunker. Life looks rosy until Jody Mills calls in. While raiding a vampire nest, Claire Novak, one of the Wayward Sisters and the daughter of Castiel's vessel was killed. The nest was far bigger than it should've been.
January 6th 2021 - Sam and Dean go on a werewolf hunt and are forced to wait before attacking. This means the wolves' prisoners bleed to death. The Mark of Cain sent Castiel into madness, and the angel is now buried in a Ma'lak box. In God's absence, the monster population has flourished and hunters are overwhelmed.
November 3rd 2021 - Dean is reluctant to carry on hunting, feeling that the Winchesters' fight is finally over. The brothers are no longer managing to save people in the face of overwhelming monster numbers and have lost most of their allies. Alternate Bobby and Jody are still alive but have a "death wish" according to Dean. Something bad has happened to Eileen, but the details aren't disclosed. Sam resolves to keep on fighting anyway and heads out to take down a vampire nest. Dean reluctantly follows.
December 9th 2022 - It's revealed that Sam and Dean have been bitten and turned into vampires. The brothers clearly haven't been sticking to a vegan diet, since Jody, Bobby and a team of hunters arrive to take the Winchesters down. Dean bites Jody, Bobby beheads Sam and the flashforward ends.
By the end of "The Trap," it's suggested that Sam's refusal to trigger the spell spared the Winchesters and their allies such a dark fate, but parts of this alternate timeline could still happen if the boys succeed in defeating God. Although Castiel wouldn't have to be consigned to a Ma'lak box, the uprising of monsters that occurs in the wake of God's fall would still have to be addressed. The Winchesters and their hunter pals would still be overrun by paranormal nasties and the unhappy vampire ending could still become reality. That is, of course, so long as God was showing Sam the truth.
Supernatural season 15 continues with "The Heroes' Journey" January 23rd on The CW.