Without speculating on his diminishing physical abilities, its sad realising that he has aged, but this is reality. There has to come a point whereby you just not putting out what you feel you want to or should be, perhaps he realises this. I know a lot of others carry on into their late 70s and even 80s but its not a benchmark for the industry, its a personal thing and everyone ages differently. You also have to realise MK has extremely high standards and always had exemplary guitar and musical skills, he is also the main focus of the show so this is much harder to maintain or mask over any cracks in your playing than if you in the Rolling Stones for example, they older but as each does only one specific instrument so its spread out, probably also less technically/physically difficult and therefore easier I would imagine to carry any weaknesses appearing.
I would be happy if he doesn't feel capable anymore and just puts out a few more albums. I am hopeful however there will be some live promo. work for the new album which is not to demanding like appearing on talk/tv shows, radio work etc. where he can perhaps do a few songs live.