I don't think Go will affect any sales of Sun & Moon.
Go has always been geared towards the more casual demographic, even those that don't own other Pokemon games. It's good as a gateway app, introducing these people into the franchise.
It's not going to work the other way around - Pokemon fans will probably get Go and still buy Sun & Moon because they obviously do different things. After getting Go, I still have the urge to buy Sun & Moon, since it's a whole new generation with an actual plotline and 3DS gameplay. I'm not saying Go is bad (in fact, I love seeing crowds on the streets catching Pokemon), but it's just quenches a different kind of thirst. If anything, it'll just boost Sun & Moon's sales. Or it won't change a thing.