Just wanted to make a couple of points with some real data.
EQ peaked at 550k players and made over a half a billion in profit for Sony, while costing $8M to develop. And the game is still running, with the rumor that there are still at least 50,000 subscribers. Now, these numbers may not compare to WoW, they are most certainly not niche. In fact, EQ was (and probably still is) the most profitable venture Sony has ever experienced.
Vanguard we had to launch 6+ months early. So even though the game sold around 250k boxes in a very short amount of time, most people couldn't play the game because the framerate/performance was abysmal. I'm not going to re-hash all of the reasons, but I will say lessons were learned. We're not going to let something like that happen again. That said, for those players who stuck around, or returned later, after the client had been optimized, almost always credit Vanguard as one of their favorite games. Even though they had to play in underpopulated shards, making grouping and community development difficult indeed.
Also, since then, WoW (and to some extent a few other MMOs) have exposed millions and millions of people to MMOs.
My point is simply this: of the millions and millions of MMO players, we are very confident that a sizable subset of them will be attracted to the type of gameplay and world we are creating. And because we are not spending hundreds of millions of dollars to build Pantheon, we don't need millions of players. In fact, hundreds of thousands would make the game hugely successful. Could we 'survive' on sub/sales that EQ brought in? Absolutely -- in fact, we would be, from our perspective, fantastically successful. And Vanguard? If 250k players try out Pantheon, and we don't screw up and the game is actually ready to be played and playable, we would also be enormously successful.
Again, if you spend hundreds of millions of dollars to make an MMO then yes, you do also require millions and millions of players to subscribe, or buy the game, or monetize through cash shops in order to be successful. But the reality is that you don't need to spend anywhere near that kind of money. Instead, you can focus on making a game for a solid target audience and have a successful product with far fewer players. You don't have to beat WoW. You don't have to spend SWTOR development costs. You don't need to try to make a game that is all things for all people. Rather, you need to choose a viable target audience and make a really good game for them.