Wilbur is desperate to recover the time machine Bowler stole, and to restore the history he sees unraveling around him, and decides that Lewis can help him. Lewis is reluctant and incredulous–the flying car does not persuade him that Wilbur is from the future–so Wilbur removes Lewis from the past, and brings him to the future.
In Back to the Future part 2, Doc takes Marty from 1985 to 2015, to help Marty’s son. As we observed, once Doc takes Marty and Jennifer to the future, they do not exist in the past, never marry, and never have that son; but then when a confused Doc restores them to their own time he restores them to history, and all of that can come back into existence for the next versions of themselves, and we can arrive at the history we see. Similarly, before Wilbur can arrive in the future all those years have to pass as he speeds beyond them, and they have to pass without Lewis. That means Lewis is not there to sell his Memory Scanner or to do any of the other things he did in the original history. A few of the major ones are that he will never invent Doris, he will never marry Fran and father Wilbur, and he will never invent the time machine. That means there is no time machine in the garage, no Wilbur to leave it open, and no Doris to mastermind the scheme to destroy Cornelius Robinson.
With Marty, none of the critical factors were dependent on what he had not yet done–Doc was born first, and Doc invented the time machine with little if any input from Marty. Thus whatever else went wrong, Doc and the time machine still existed. Lewis does not have that advantage: the time machine only exists if he invents it, and its operator only exists if he marries Fran. If he travels to the future, he makes it impossible for himself to do so, because Wilbur will never exist to bring the time machine to him, which also will never exist without his inventive successes. Indeed, Bowler and Doris cannot be in the past, the latter because Lewis will never have invented her, the former because there is no time machine for him to steal and no successful Lewis for him to envy.
Having undone all of this, Wilbur ultimately undoes his own trip to the past and correspondingly undoes Lewis’ trip to the future. That leaves Lewis in the past, and allows him to sell his first invention and continue from there to become Cornelius Robinson, marry Fran, invent Doris, sire Wilbur, invent the time machine, be envied by Bowler, and so ultimately be taken to the future. However, once again as soon as he leaves the past he makes his own trip to the future impossible–and since Wilbur is taking Lewis to a time after Wilbur left the future and now cannot have left the future, they again will never arrive. That time does not exist. We have an infinity loop, each history causing the other.
When the robot said that there was a 99.999999% chance that Wilbur would not exist, he was overly optimistic.
The movie is not yet finished even though it has managed to finish time.