I have no problems with a population of that size, provided the game world isn't designed for 10,000 people to exist in every zone. I mean, seriously, we talk about WoW's population and somehow we get that mixed up with some belief that you as the player were literally interacting with millions of people in WoW. You may have interacted with potentially 2,000 people in total on your server. Seriously.
The only people that should have problems with a well-designed world, that has plenty of activities to keep players busy through various means, but doesn't have millions of active subs, are developers themselves. We, as players, don't need that many people playing the game. It was once thought that having more players would equal more expansions, but that isn't true. WoW has always chugged along at its same pace in terms of content updates and expansions. I doubt that schedule would change whether there were 30 million or 300k people playing the game.
So long as a server isn't struggling to fill slots for even the most basic events (like a ballista match constantly being canceled because 6 people for both teams can't be bothered to show up), total population figures are simply something for shareholders and the media to drool over. There has never been a MMO world designed for literal millions of players to exist on a single server, and there never will be. MUDS don't count. Text-based games don't count. Shitty graphic games that house "thousands" of players within a single vehicle don't count.
Edit: I'll make this text big and bold so people that love "defined role" games can comprehend this: Defined roles are great until the developers of a game provide evidence that they have fuck all of an idea how to properly define the roles of a class within a game. Yes, defined roles were great when/if you were a bard, ninja, bard, red mage, bard, ranger, bard, black mage, or bard, but they weren't so very nice if you were a drg, cor, bst, thf, or drk. I only want to see defined roles when they are properly done, and even the word "properly" has a mixed connotation, as games like WoW, with large amounts of jobs, wound up tossing many of the defined roles across multiple classes, just to make sure nobody was sitting out in the dark while every other class got to have fun, which was the case with XI.
Hell, SE still hasn't shown that it knows what the fuck to do with certain classes aside from granting them one decent ability and tossing everything else to the wolves with XI, and XIV isn't shaping up to be too much better at this point.