Hm, interesting topic.
We're all aware of those less guitar driven songs but...I'm not sure is it has to do anything with age, in Mark's case. Following that logic, guitarists like Vai, Malmsteen etc etc, should not be able to play 90% of their repertoire after they get over 50ties???
Mark was never that fast, if we can call him a fast at all. But that's not the point. The good side of it and also the reason why i think he will be able to correctly play any of his song for a long long time, is exactly that balanced amount of speed, soul, and technique in his playing.
Maybe he just thinks that if you put more guitar into song, you loose on other (more important) aspects of the song. I personally would disagree with that, but as he said himself - "guitar is just the thing i used to write songs with it".