I could imagine that globally the most common replies are three forms of "yes" but for any individual user the most common replies are two forms of "yes" and one "no", simply because there's less diversity in one user's replies than in the entire population.
That this the only other choice available is also significant, particularly in reference to other cases where they have chosen to make turning something off impossible.
Taking away the extremely popular "classic layout" is an example. It wasn't costing them anything, but getting people used to accepting arbitrary changes forced on them has worked out very well for Apple, and Google has to want some of that. Each passive acquiescence makes the next easier, until it becomes automatic.