Alright, let's take a closer look at this:
The quote:WoW released in Nov 2004, and 5 years later, they did that quote as well as the quote that they believe that the majority of people who was going to play WoW already played WoW. So that means in 5 years, they were losing 6301 players per day (11.5M/365*5) at its most conservative amount of players. If it is indeed closer to triple, they were losing 12602 players per day AT THE MOST. However, during this time, they had a huge draw for players. It was a very popular IP and was had a low gate for trying out the game (only had to buy WoW box which was $40). So during this time, they felt the majority of people who were going to play WoW (let's use the ages of 10-70, we can actually use 5-70 due to being 5 years later). Assuming that quote is true, this means that there is a small minority between ages 5-70 that did not play WoW that did want to play WoW and then we have the new gamers who were just being born to age 1-4 at the time since this only 4 years after that. That is A LOT smaller user base to try to draw from. The entry barrier is the same as WOTLK (two expansions and battlechest so 3 games) BUT it is now 90 levels instead of 80. For those wanting to play with their friends, this would reduce the incentive of wanting to play.I don't know what the exact number is off-hand, but the total number of subscribers we've had is easily more than double - maybe closer to triple - the current subscriber base."
Also, let's say they do give the trial a try - the game is 9 years old so the incentive to teach and be friendly to new players is lower than it was 5 years ago. Hell, they even tried to introduce a guild program for guilds that would be helpful to new players and recommended those servers on new accounts.
From there, there has also been a massive amount of changes over the years (doesn't matter if they're good or bad, they have occurred). For players who were interested in coming back, this adds an additional barrier that can detract from players coming back.
Now, let's talk about competition. At release, WoW was the CASUAL of MMOs that were available. There was Everquest, Eve, Guild Wars 1 and a few others but basically, games that were much more hardcore and also used the subscription model. Let's take a look at the current market: We have SWTOR (very large IP, and F2P...even though it is a shitty model), Guild Wars 2 (B2P), Tera (F2P), Age of Wushu (F2P), Rift (F2P) AND many others, including Everquest and Eve. So for someone who was just interested in a MMORPG, they have games that offer a lower bar for entry and offer a comparative experience (in some ways).
Let's take a look at what has occurred with WoW since 2009: It peaked at 12M quickly after this announcement, then started dropping during this same expansion to 11.5, peaking again in 2011 for Cata. Since then, it has dropped to 7.7M losing 4300 people per day. So all this says that the 9 year old game is still losing people per day, but not bringing in as many due to the many reasons above. I'm sure if I spent some actual time doing this that I could come up with a hell of a lot more reasons.