I noticed that the guy who said this is the creative director of Assassin's Creed III, a game that is the poster child of everything wrong with with Ubisoft sandbox games. Mainly pointless collectibles and no sense of cohesive game design. Watch Dogs also suffered from this issue.
Personally, while it is possible to make a really good sandbox game, it is easier to make a tight game when it's linear and you don't have to feel obligated to put pointless sidequests that have no real purpose other then dangling keys to swing in the players' faces. I like a good sandbox game too but saying that linear games are going to become completely irrelevant is kind of ignorant and an oxymoron.