It’s something of an understatement to say that the reception to Jensen Ackles’ Supernatural spinoff The Winchesters has been decidedly mixed. Beyond the dubious validity of the story even being told, it also requires a significant retcon to even exist.
At the very beginning of the series John knew nothing about the supernatural, and was only inspired to research and eventually fight back against it after Mary was killed by Azazel, the Yellow-Eyed Demon. Since then, the show’s mythology was continually developed and expanded, eventually revealing that John was a legacy of the Men of Letters. However, his lack of knowledge about the underworld that would eventually consume his existence and claim his very life is the central problem.
The premise of the prequel is that it’s an epic love story seeing John and Mary risking everything to save the world and will presumably involve them battling monsters side by side, which on paper makes no sense when continuity demands future John to be unaware that anything otherworldly goes on when the sun goes down.
Of course, such an issue has actually been dealt with previously, in time travel episodes set in the ‘70s featuring young John and Mary. In “The Song Remains the Same,” the final of these prior to the present, John’s memory was wiped, thus justifying his having no recollection of any paranormal encounters prior to his wife’s murder.
There’s no reason that something can’t happen to restore John’s knowledge and experience of monsters for the sake of this show, but it still leaves the same problem: by 1979 at the latest (Dean’s birth) John can’t know anything about the creatures of myth that stalk the night preying on the unsuspecting, while Mary has to pretend she doesn’t. This means that something has to happen to make John lose his memories (again) if The Winchesters wants to tie in with its parent show.
It’s possible that the creatives behind the Supernatural prequel will manage to pull off something unsuspected and deal with the continuity snarl in a satisfying way, but if holes are being poked in its premise this early it doesn’t bode well for fan acceptance when fully realized.