As long as there are people who will pay for EverQuest, it's going to stay online. That's just a fact of business, as SOE will continue to develop new and engaging content to ensure that they can moving on up in the corporate world against their competitors. Sure, it doesn't look as pretty as the newest MMORPGs, but it sure does have a very loyal following - 400,000 strong still. That's nothing to bat an eyelash at!
Also, EQ is certainly not the game that has stayed up for the longest. The first graphical MUD - called Baram (in Korea, or Nexus in North America;
http://baram.nexon.co.kr or
http://www.nexustk.com) just passed the ten year mark. I believe that Baram's been online for 11 years now. Yet, Nexon (the developer and publisher) continues to have a very loyal following to that game as well - with about 100,000 subscribers. Nexus itself is holding 4,000 accounts. Or something like that. Mind you, this is in Korea, where people would have gang fights in real life over things that happened in Lineage. Neither of these games are as popular as EQ is - there's a reason it's called "The Most Successful Massively Multiplayer Online Fantasy Roleplaying Game Ever!"
Though you might notice as time draws on, the quality and originality of these expansion packs is likely to drop. They'll still charge the same amount, and the same people will continue to buy them. To the die-hard fans - all of them, and there are a lot of 'em (just look at how many people pay for EverQuest Legends) - won't care if they've done the same quest over and over. Nor will they care when they go on their Epic 17.0 quest in 2012. People play games because they like to, and EQ has a unique way of keeping players coming back for more. Even if it is cliche, and the same stuff, over and over again, year after year.
But let's take this a step further. Let's say that there are 400,000 active accounts for EQ, each paying... $13.95 per month to access the service. Doing some quick math that's $5,580,000 per month revenue to SOE - or $66,960,000 in revenue per annum (and that's just access fees). And that's to use maybe 150kbps of data transfer and no more than 3mb of server space. If anything, SOE can even widen their profit margin by consolodating servers as to eliminate the fees associated with running one of them. I can't see any possible way - from a business perspective - to lose $66,960,000 in annual revenue. That's too much, and it would probably have SOE shut down most of its services. There is no denying it, EQ is a huge cash cow for Sony.
MMORPG players have a very well-known history of sticking with a game for months and even years at a time even though the game itself sucks. Or the customer service sucks. Or whatever. But players are attached to the game itself and the community and that desire to keep in touch with their friends that they meet through the game. That's why EQ continues to be successful. If everyone in EQ decided to jump to EQ2, then the friends of those people would make the jump too. It's a typical social networking scenario. MMORPGs' content is covered under copyright and such, and also every company's probably got a stack of patents just for the game itself. EQ is no exception. In the free market, one cannot jump to another company to play the same game because that other company cannot exist. This isn't like when you're dissatisfied with the gas company you're buying your gas from (in that case, you just go to another company, right?).
So as I said, EQ will continue to be online as long as there are people out there who will pay for it. Even if there are 2,000 accounts worldwide, there will still be one or two people working on the game. These games are never really written with an ending in sight. That's the beauty, and the curse, of the MMORPG.