I am not sure if I would continue gaming were single player games to become truly all that rare.
As others have pointed out...there are plenty of single player games that have made their developers/publishers a lot of money. Many, if not most, become full on franchises that basically print their own money with each subsequent sequel. Hell, I don't even like Assassin's Creed or Dead Space, and yet any idiot can see that they are obviously doing quite well being primarily or only single player games.
But his line of thinking bothers me mainly because the two things I see hit the hardest, in terms of game design, when a game becomes focused either partly or entirely on multiplayer is the storyline and the character development. I don't just mean shorter single player campaigns or limited hair options either. I mean all of it...things like the writing, voice acting, etc.
For me, my favorite games have always been story driven with a player character that I was allowed to make my own. While some MMOs are good at one aspect or the other, I have played very few that are good at both...and none that were able to stay good enough to warrant a subscription. The other thing is that the two main issues he states are pirating and renting...and quite frankly I don't see how simply being a multiplayer game changes any of that in a way that a single player game can't.
It would definitely negatively affect my interest in this hobby if this man's opinions turned out to be realities.