I think subs are just evening out, just like all WoW clones cut from the same cloth like ARR is. Active subs will most likely hover around the 500k mark and go lower or maybe higher at times, regardless of how many accounts and "adventurers" there are.

Thing is most MMOs don't need millions of subscribers to stay afloat, just look at XI with their lower than 500k subscribers still getting substantial updates every month, not just every 3 months and their monthly patches have more content than ARR's somewhat monthly 2.x5 and 2.x8 patches. Legacy servers will obviously always be the most populated, because of the people playing on them since 1.0. Most people will have an attachment to their characters and the game by now and will stay around for a long while.

That core group of loyal players is what drives pretty much every MMO regardless of their age, financial model and server populations, and of course you also have the usual minor band of apologists who'll defend anything and even if a game suddenly started charging $1 per dungeon run they will call you entitled for not wanting to pay.

Whether this inevitable drop in players will be good or bad for the game will depend on how they handle it. XI has been doing great adding and tweaking a lot of features and content to what a couple of people around here would otherwise call a dead game, even Everquest is getting its 21st expansion soon. There have been very few MMOs that have completely disappeared and failed, so it's very likely ARR will still be around even with a huge drop in subscribers, for better or for worse.