As a scientist I can tell you that you can certainly make claims with differing amounts of data. What changes is the precision of those claims. For example, I can compare two objects and say that one has a higher temperature than the other even without a preponderance of data. I might not be able to tell you the temperature difference to a very precise value but I can note there is a relative difference.
Similarly, we can observe the situations I mentioned and note that there is less player activity in them then there used to be. Servers have less layers, there are less BG instances forming, it’s more difficult to form groups for most content, guilds are shrinking or disappearing, and so on. This is not a comprehensive survey and we don’t have exact numbers but we don’t need those to simply note a trend.
To deny that there’s such a declining trend is to be willfully ignorant of what’s going in in the game. I’m sure that it will have a long tail and that the game will be playable at some level for a while but the experience is greatly degraded in regards to being a cooperative-type of game.