Originally Posted by Jennifer
But Old Doc isn't actually from 1986. He can't just fade into his 1986 self. That's not how it works in the Back to the Future universe, or else Marty would have faded into Switzerland Marty in Back to the Future II because Marty was two weeks older than he should be. Citizen Brown is the 1986 Doc. He already exists. 21st century Citizen Brown doesn't exist, so Doc can't either.
It doesn't mess with the message of the future isn't written, because it still falls within the rules of the Back to the Future universe. Doc's message to Jennifer and Marty in Back to the Future III is more like the future in any particular timeline already exists (or else there wouldn't be a Marty and Jennifer in 2015 in Back to the Future II), but it can be changed since they haven't lived it yet.
The reason Doc faded is a matter of the time ripple effect. The ripple effect catches up to people faster if they go past the time period where the error that caused them to not exist had occurred. Because Doc went past 1931 (where the changes to his life occurred that caused him to not live into the 21st century), he couldn't beat the ripple effect to save his life, just as Biff couldn't beat the ripple effect when he went past the 1990's (since Bob Gale said it was intended that Lorraine killed Biff in the 1990's in the Hell Valley timeline).
I agree...it makes perfect sense for Doc to fade because his timeline was drastically changed after 1931. What we're trying to make sense of here, is why did Doc fade and Marty and the Delorean NOT fade?!!
In the Hell Valley timeline, and the Tannen Mob ruled timeline in Ep 2, both Marty and Doc didn't fade. Even if you assume the Citizen Brown timeline messed up things SO badly as compared to the other two timeline, then why didn't BOTH Marty and Doc fade?!!
Honestly, the only workable theory IMO is that Doc is fundamentally different from any other time traveler in that he's the inventor of time travel, and so changing his past critically tends to scr#w things up more and have unpredictable effects!