Here is the post that should end this discussion lol. =P
What people somehow miss to realize and notice is that Parkinsons didn't just affect Michael's phsysical movement(which people are obviously convinced about that it shouldn't hinder voice recordings which I believe to still be a short assumption) but also heavily his speech. Look at any interview. Yes he can talk, yes he is easy to understand but has he ever produced a clean sentence? No, he has to stutter he hampers there is irregularity in his tone.
To do a recording of a game which is heavy on dialogue, it's going to take him too much effort and energy to deliver the lines in a way he feels need to.