Now I will type this really slowly so you can understand.
Yes, the individual meetings shown on SKY may have a decrease in crowd numbers, not always the case tough as has been proved by Lakeside and Swindon if promoted properly they can still attract decent crowds.
But the overall exposure the sport receives by being shown on SKY can only be a good thing for Speedway on the whole. Casual viewers and lapsed fans (myself included) have been brought (back) into the sport by attending at their local tracks.