Do you mean the DeLorean that Edna is driving? It was stated that since the timeline that Edna's DeLorean came from was written over, the molecular composition of that DeLorean was decaying at an alarming rate.
Once the new timeline caught up with itself, that DeLorean ceased to exist since the Doc that came back to get Marty was a completely different Doc than the one that built that time machine (since the new Doc lived in a timeline where his inventing was supported by his father up until the day that his father passed away).
Basically, the reason the old Doc had disappeared when him and Marty jumped to '86 at the end of Episode 3 was because they had hit the point of no return, aka the point where Doc had lost all interest in science, had gone in full with Edna and basically, the point where there was no possible way for the original Doc to still exist in the altered '86.
The reason both the DeLorean and Marty still remained unaltered was there was more or less no point of no return for them. There's always the chance that the DeLorean can be turned into a time machine, no matter how unlikely, there's still an extremely slim chance, which is why a few of Doc's notes doesn't disappear either, the ideas can still happen to Doc, only they are transformed into different things, I.E, the Citizen Plus plus logo.
As for Marty, there's always the chance that his alternative self could wise up and begin becoming more cooler, so there was again no point where Marty would vanish.
There isn't any other alternative reality Marty because he basically overwrites Marty because his own personal timeline hasn't changed too much that it's impossible for the current Marty to exist, unlike Doc where his timeline was so radically altered that there was no chance the old science loving Doc could exist.
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Why the original Delorean dissapears at the ending?
I didnt understand that. Thanks
Once the new timeline caught up with itself, that DeLorean ceased to exist since the Doc that came back to get Marty was a completely different Doc than the one that built that time machine (since the new Doc lived in a timeline where his inventing was supported by his father up until the day that his father passed away).
Thank you
Basically, the reason the old Doc had disappeared when him and Marty jumped to '86 at the end of Episode 3 was because they had hit the point of no return, aka the point where Doc had lost all interest in science, had gone in full with Edna and basically, the point where there was no possible way for the original Doc to still exist in the altered '86.
The reason both the DeLorean and Marty still remained unaltered was there was more or less no point of no return for them. There's always the chance that the DeLorean can be turned into a time machine, no matter how unlikely, there's still an extremely slim chance, which is why a few of Doc's notes doesn't disappear either, the ideas can still happen to Doc, only they are transformed into different things, I.E, the Citizen Plus plus logo.
As for Marty, there's always the chance that his alternative self could wise up and begin becoming more cooler, so there was again no point where Marty would vanish.
There isn't any other alternative reality Marty because he basically overwrites Marty because his own personal timeline hasn't changed too much that it's impossible for the current Marty to exist, unlike Doc where his timeline was so radically altered that there was no chance the old science loving Doc could exist.