Originally Posted by Michael J Fox is Canadian
Well there is no proof an alternate marty still exists. In fact there's proof it doesn't work this way; the eastwood timeline is an alternate timeline for marty (who is from twin pine). He's been there for 6 months and there's clearly no eastwood Marty lurking around.
There is proof the timeline can predict the future without time travel; in part II when Marty burns the almanac, the newspapers from 1973 and 1983 immediately change. Not unreasonable at all to predict FCB does not live to age 90.
The Eastwood timeline MAY be an 'alternate timeline' in the technical sense, but it differs very much from the kind of alternate timeline we're discussing here (Citizen Brown, Hell Valley etc.) in one very pertinent way...the fact that Eastwood Marty, UNLIKE FCB Marty or Hell Valley Marty, time traveled. From Doc and Marty's POV, it doesn't matter what changes...as long as a few basic facts stay the same (Doc coming up with the flux capacitor, inventing the time machine, becoming friends with Marty, the experiment at the mall, Marty getting sent back to 1955 etc.) happen, they still consider it the 'normal' timeline, albeit with a few changes.
There is no way Hell Valley Marty OR FCB Marty ever time traveled. So, if you claim there is no alternate Marty, then the question arises...WHAT happened to the alternate Marty? He clearly existed...people in the FCB timeline have memories of him...did he simply cease to exist the moment 'our' Marty showed up in 1986? What sense does that make really? If at all a Marty was to be erased, shouldn't it be 'our' Marty? (since that's clearly what happened to 'our' Doc)
As far as your second point goes...true, the timeline has always been able to predict the 'most likely future' based on the current status quo, but consider the kind of predictions it has made. The timeline has 'predicted', as of November 12th 1955, that without Biff having the Almanac, George McFly will be alive and well in 1973 and win an award. A plausible enough prediction to make, given the current status quo (George having just gained confidence by punching out Biff is likely to become a writer, and Biff can't do a thing about it!) But remember, the only reason this prediction has manifested itself in a material form is because a material piece of the future is in the past...namely, the newspaper. The newspaper is intrinsically 'linked' to its time period of origin and as such is a window to what will happen at that point of time along the current timeline. Its not the timeline itself making the prediction. Also, other predictions, like the probability of Bufford killing Marty in a duel, or Griff being arrested for vandalising the Courthouse are pretty straight-forward predictions to make. But predicting the total life span of a person, given the MILLIONS of factors that affect that, is another matter entirely...
What I'm trying to point out also, that its not the continuum ITSELF predicting the future, as much as objects linked to the future by virtue of their origin giving a window to a certain time period...