Final Fantasy started its life as a turn based RPG on Nintendo. 20 years later while the graphics have stayed cutting edge, the gameplay hasn't dramatically changed and it's still a turn based RPG with random battle encounters and predictable combat. Only with the last iteration (FFXII) in 2006, did they significantly modify this formula.
In the meantime, this franchise has produced ~30 games (including a MMO), with only 11 in the main flagship series. In addition, there's been 2 movies, soundtracks, countless anime, manga & graphic novels, and all the accessories a fanboy could ever want. Notice they recently opened an online store *drool*.
Final Fantasy is one of two reasons why Square Enix is such a force in the Japanese offline RPG space (the other being Dragon Quest). However, to look at game sales by itself would not be completely fair. Square generates substantial revenue from action figures, playing cards, accessories, and publications.
Glancing at their 2007 projections you can see the Square-Enix projects ¥9 billion for offline game sales (traditional product) out of ¥21 billion total sales, a little less than 45%. Slowly but surely, what started out as a console game hit, has morphed into one of the most recognized franchises with multiple touch points.
None of this would have happened had Square-Enix not started with great storylines and well developed characters. They made the audience care about what happens to their characters. When Aerith dies in FFVII, the player feels remorse. You genuinely feel a loss, despite the fact your party still has plenty of replacements. What's more impressive is that Square "starts over" with each title. To date, only one game has been a direct sequel in terms of continuing storyline (FFX-2...which btw is the worst FF ever. You "change dresses" to access different powers --> Extremely emasculating). Each game has a different set of characters in a separate universe, unlike Mario, Zelda, Sonic, Kirby, and Lara Croft, who have had the benefit of several titles to increase user attachment.
Square can not only compel their audience to spend $50 for each game but can also up sell some rabid fans to pay $150 for a resin statue of a character in the game (yes, I'm guilty). I have probably spent $300 on FF games but $500 on the toys. The film business underwrites movies to break even in the box office, and generate profits through DVDs, premium & broadcast TV, digital downloads. It's possible to see video game companies spending a good deal of money to incubate strong IP that can be leveraged onto other channels and business lines.
But wait, does story always matter? Obviously if you're producing a game primarily played in multiplayer mode, then the entertainment of the game is in participating in competition with others, not moving the story and characters ahead in a directed experience. This is why Halo 3 can have one of the lamest stories of 2K7 and still manage to sell over 8 million copies in a few months. Counterstrike and Team Fortress are other examples --> they're both mods of a game, so there's no story altogether!
Some claim that story and character development only matter for RPGs. I disagree. Tomb Raider is an adventure/action game and Lara Croft is probably one of the most valuable characters in the video gaming industry due to her distinct personality and look. The recent Grand Theft Autos have focused on developing the main character. Story and character development can even help reinvigorate simple casual games. Puzzle Pirates and Puzzle Quest, are all glorified versions of Bejeweled when it comes down to it. Shooters such as Half life and Portal don't need brilliant stories to stand on their own, but have done phenomenally well by incorporating them into the gameplay.
Fact is that most games weave in the best elements of other genres. It's becoming difficult to define games by a single genre, "that's a first person shooter" or "that's an action game". Games no longer operate in silos; the best games combine parts of many genres. In particular, newer games are borrowing heavily from the realm of RPGs by developing their characters and placing an emphasis on involved storylines. Given that storylines and characters are much cheaper to develop than impressive graphics, I'd say it's an investment worth making.