Not sure how we got from Jen's extension to Geno and his retirement date, but I shall plunge on.
I actually think Geno loves coaching - he loves figuring our which buttons to push for each player, and he takes real pride in the development of their skills and great joy in their success. And his players become his extended family. He is Italian and so he loves huge families, and what bigger family is there than 28 years of players and assistants flung to the four winds but always coming back for visits. And he treats his teams like family, bringing them into his home and his real family, feeding them, and providing an 'in loco parentis' environment. He is the 'godfather' of his little athletic kingdom. I think retirement would be devastating to him. As long as his health and his energy remains good, and he continues to like and enjoy his players, and he has reasonable success, why quit. What would he do with his time - he isn't going to sit around the house. And i am sure he enjoys the food and wine business - it too is a social environment, but i doubt it really satisfies him the way the coaching does.
So ... based on that reading of him - if he is healthy at the end of this last extension, I expect another extension to follow.
And prior to when he retires I think he and the AD will devise a succession plan that will involve hiring his successor and working with whoever that is for a year on longer.
As for the successor, it really does depend on the timing. I suspect the front runners would be his coaching tree - assistants and players who have gone on to HC jobs elsewhere or who are on his staff when the decision is made. But it could also be someone outsider like Jeff who he has coached against and grown to respect. I would be surprised if the AD at the time did not hire whoever Geno recommends.