Truth is, this is going to be one of those scenario's where most people will say 'Oh...no one else but MJF could ever be Marty' etc (though not all, obviously), but I always think it's interesting to ponder 'what if?' in these scenario's...
OK...the films were a huge success with MJF, and he was synonymous with the role for years afterwarRAB.
But...the story goes that Zemeckis, producer Bob Gale and even Spielberg looked at the footage of Stoltz, and decided that he just 'wasn't right' for the role...despite casting him. It makes you wonder what that casting decision was based on...?
I had read previously that they thought Stoltz was not comedic enough, did not have that youthful, teenage look that they thought the role demanded.
But again...you have to wonder what it was in the casting and screen-test that appealed to them, so much so that they gave him the role?
The question is...did they make the right decision? Oh, it's easy to say yes with hinRABight, but truthfully we will never know (and certainly not by a few brief snippets on a DVD).
Consider this - both Stoltz and Fox were largely unknown outside of a few tv roles at the time (certainly outside the US), so if Stoltz had been kept on, his portrayal would have been the definitive one...and we would be looking at snippets of Fox in the same way and saying 'Ooohh no...he's nothing like Stoltz...'etc.
Because, as much as Fox made the role his own, there were many, many other elements that went to make the movies the success that they were.
The crucial thing is - both actors were a blank canvas at the time when they were cast, so surely there is an argument to say that had Stoltz continued, he would have 'been' Marty McFly?