True people come and go for a lot of reasons, however that is a constant rate of change. Peaks and valleys in something like WOW are caused not by the constants, but by things specifically dealing with the product.
Example: Coca-Cola
As a product it has a constant rate of sales growth, it nearly matches the rate of growth of the human population. However people stop drinking coke often, and for a lot of reasons. That is all accounted for in the long term growth curve. So if for any reason a sharp decline in sales happens around the time you stop selling Coke and Sell "New Coke", is it the economy or is it the change in flavor? (look at sales in 1985)
When new Coke hit, sales dropped to a 5 year low, as soon as they released 'Classic Coke' Sales skyrocketed.
You can use the Coke lesson with WOW and see that the same thing is happening old WOW sold well and had a constant rate (this includes people coming and going for what ever reason), the Cataclysm changed things, the Growth curve became a downward trend, Mop came out and it became a sharp drop.
To someone who cares about the game and the quality of game-play in the game, seeing a sharp drop after 3 years of growth, and 5 years of consistency. That is a concern. Sure Blizzard is still making money, they are offering many online services, and they have other games. As a company Blizzard is a good investment, however I am concerned with the health of WOW, not because it is an old game, but because it is hemorrhaging subscriptions. And the only factor that can explains the current losses relates to in game. Game-play changes, and Content.