"PC gaming is dying! Console gaming is doomed! Adventure games are dead! 2D games are gone forever! The fantasy genre has shuddered its last fetid gasp! The monthly subscription business model is destined for extinction! Flight simulators, we hardly knew ye! The tactical RPG: gentle giant of a lost golden age!"
Supply and Demand
I don't know where these people were on the first day of freshman econ, but as long as there is a demand for something, it will be profitable for somebody to produce that thing. Demand may increase or decrease over time, and the market may get tougher, but it's very unlikely to disappear.
Smart companies are going to produce games in the genres that have the least competition. Say there are only a few hundred thousand people in the world who want to buy adventure games, but there are practically zero adventure games being released per year. A smart company with a well-placed game that's reasonably good can practically guarantee themselves some decent sales.
There are some very smart developers out there, who've set themselves up with presumably 100% of the market share for some incredibly specific game genres: according to their homepage, 3 Rings has 3 million people playing their massively multiplayer pirate-themed puzzle game, and 500k playing their massively multiplayer wild-western tactical rpg. Why make another fantasy game when you can corner the market on a new genre that you've just invented?
The Gold Rush
Every time some smart person makes a boatload of money on a new demographic or genre, there's a huge subsequent rush and glut of content in that space. This happens in every medium: Look at CG family films post-Pixar, comic book movies after Spiderman, e-commerce websites in the 1990's, or children's fantasy novels after Harry Potter. Eventually, there are enough people jostling around for dominance in the space that it's no longer a license to print money, and the market will start to stabilize and fall off, sometimes drastically.
Right now, the glut of fantasy MMORPG games is driving people to start looking for opportunities in other spaces. But imagine twenty years from now, if all those other genres become so successful that the fantasy genre is pretty sparse. Someone smart will decide they should make a great fantasy game, retire early, and spark a whole new boom of fantasy games.
These types of cyclical changes in supply and demand happen all the time. Celebrities make too many movies and start to annoy people, only to lay low for ten years and then make a comeback. Music and fashion go out of style and come back in again. Conservatism and liberalism come in and out of power, as one group takes things too far and people rush to support the opposing side.
This is how the world works, people. Enough with the doom and gloom.