I just don't believe that "he can't play" the fast songs anymore, I have just re-watched the Soundbreaking documentary again, which was only from 3 years ago, he was playing some fast stuff then, but I feel it is a combination of Mark putting most of his efforts towards songwriting more than playing the guitar plus age related health issues (arthritis maybe?) that is the factor, he has been playing as a professional musician for most of adult life, so you don't loose that ability he has, yes it can go rusty, but the ability and desire to play that way can and does wane. Mark is in a very lucky position in that he can do it all to a very high standard, he doesn't just have to rely on his guitar playing to get him through, like Eric Clapton for example, Eric has to keep his live playing good, because he can't fall back on great song writing, same with someone like Joe Satriani. I think this is backed up by the fact he said he was looking for a Jazz teacher, although he may have been teasing with that
Another big factor I think is since Chet Atkins died, Mark doesn't have to keep his playing to a very high level to keep up with Chet, for Mark, Chet was as good as it got for guitar playing, and after Chet there is nothing for him to prove anymore.
