I agree that the sequels are lesser films, and Michael J Fox as those other characters certainly doesn't help them.
And the comparison with Coming to America isn't really valid. Eddie Murphy (and, to a lesser extent, Arsenio Hall), were all wearing makeup and playing completely different, unrelated characters (indeed, Orbert couldn't even tell). Not the case with the BTTF sequels.
The first BTTF is magical, and one of my favorite films.