Was Stoltz replaced with a time machine?
Somebody learned at some point that the movie was not going to work with its original cast. So they hired MJF and everything was fixed.
How did they figure with such certainty that they had to do this?
Stoltz was not THAT bad. Yet they were sure that was the right move and went through such a messy affair.
I think it is clear that the time machine was used to go to 1987 and see BTTF flop miserably and MJF become super famous starring in something else, like Ferris Bueller's day off.
So they knew what had to be done. We are living that alternate timeline.
Question is, who used the machine?