Genuine question.
He is an old man (in F1 terms).
However, he has incredible marketing power. No doubt about it. He also has a massive fan base. Any lesser team who needs money and a decent driver (but can't afford a great driver), would certainly take him on. Sauber have a seat available, so he could be an option.
And as for MSc going to Marussia - why not? A driver who is on the brink of retirement with his best years well behind him might find the Marussia team attractive, with a view to helping them develop a car and then moving into management or even team ownership (something that is possible, with a small/newish team). MSc has certainly reached an age where he should be thinking about going into management or running his own team. If anybody can attract talent (eg. drivers, engineers, designers, etc) to a team, still finding its feet, it would be MSc. MSc remains a huge name in motor-racing and I doubt he will squander the opportunities which may come with that name.