Honestly it's something I'm going to have to watch more and pay closer attention to because I want expecting this sort of boost to full game sales. That's something credit to
MatPiscatella who said he could see that happening prior to the service launching.
I was thinking Game Pass would either cannibalize sales or at best would have a mild positive impact on sales. Certainly not the huge boosts we are seeing. I though Game Pass would be a huge success but primarily due to it driving recurring subscription revenue, not boosting full packaged and digital game sales.
My hunch is a rather simple one in the end. Exposure. Game Pass puts the game potentially into far more people's hands than it other wise would have. This is going to convert some people into purchasing it outright that are already GP subscribers in case they eventually unsubscribe. It's also, probably more importantly, having those GP subscribers inform their friends about the games they are playing and they are then going out to buy the game themselves, even though they aren't GP subscribers.
All in all it's working wonders and FH4 has the potential to be seriously huge imo. Especially with the expanded multiplayer / shared world features