As others have pointed out, if the cabin is full then most likely, everyone paid more or less what they did (or indeed, often considerably more) and so some people will fatally not have seats together when they are down in the status and fare pecking order (since full fares get free seat allocation). If seat selection was free for all, the result would be exactly the same in terms of some people not sitting together, except that it would be based on when people book rather than their value to the airline.

That said, it is not unlikely that they will manage to sit together, although likely only in the middle section rather than the highly desireable window/aisle pairs.