It's not hard or expensive to come up with an interesting world, a compelling story, or memorable characters. Anyone with a bit of a creative writing background and some time on their hands can take a half-assed stab at that. What's hard and expensive is building a game client from the ground up, designing a cohesive art style that's both functional and attractive, managing a multi-year development process with dozens of parallel projects occurring at the same time, and getting it all to work together within the scope of the time and budget allotted by your limited resources. In short, it's actually building the game that's the hardest part of making an MMORPG. At the end of all your labor, you might still end up with a piece of crap that no one wants to play (because you can't test it until you have a finished product).
That's why having a code base is so nice. If you have a code base, you get to cut through a lot of that crap and go straight to the fun stuff -- tweaking the world to fit your vision and making interesting content with pre-established tools. It was that easy start-up that gave rise to so many MUDs in the mid-90s, but we don't have anything similar for MMORPGs today. Everyone who builds an MMOG starts from scratch with their own technology. There are a few attempts to get a universal code base to the market: Hero Engine, Metaverse, and Big World. Many companies are looking into these platforms for tech solutions. However, they haven't really been tested in the market yet. Frankly, we don't know if they lead to fun games or not. Will combat feel stilted and awkward? Will the questing or inventory systems be foreign and strange? Maybe. Maybe not.
We do have one example of a game that's proven itself for many people to be hugely popular, fun to play, and has intuitive and comfortable systems of combat, movement, and questing, however: Imagine what might happen if Blizzard decided to start selling their World of Warcraft code base along with a suite of tools for content creation. Furthermore, imagine if there was a scalable license that let everyone from the smallest indie to the major corporations use it (a cut of final profits made, perhaps). Suddenly, everyone could stop trying to clone WoW. If they wanted it, they could just build off it. I think that there are a number of comparisons you could draw between a WoW code base, if released, and the Diku code base (ignoring the fact that Blizzard is not a small group of open-source friendly programmers).
It's not outside the realm of possibility, either. Blizzard's strengths are in their polish, branding, fun content, and marketing know-how. Releasing the technology behind their uber-popular game wouldn't really damage their competitive advantages too badly. Besides, people can already see and copy the game improvements they've made to the genre. You could even argue that it would strengthen their position, since World of Warcraft would still be on top and all of the people trying to compete with them would also have to compete with a whole host of similar indie projects that could field comparable and accessible game technology (thanks to a fairly easy out-of-box setup). Furthermore, there's a precedent for major companies doing something similar: In 2000, Wizards of the Coast performed the pen and paper equivalent of this by releasing their d20 system under the Open Game License, allowing interested third parties to build on their work. Blizz could just slap a "powered by Blizzard technology" label on every game that used the code base and take a cut of the profits. Talk about an enduring legacy!
Finally, it would give amateur players and coders a chance to really flex their creativity and show us what they're capable of. Since you know the engine is fun to play in and the licensing fees wouldn't kick in unless you made money off your work, the risk associated with trying new things and making dramatic changes to the code would be small. In the spirit of open-source software, everyone could cherry-pick the best ideas and adapt them for their own games, making the overall diversity and quality of MMOGs shoot through the roof (without bankrupting anyone). Nobody would play the sucky games, and the good ones would differentiate themselves with content and build dedicated groups of users that would be willing to pay for continued development and customer service. Iron Realms Entertainment still makes money to this day on text-based MUDs thanks to the extremely deep and high-quality nature of the gameplay -- they took the MUD concept that was out there and polished it to an unparalleled level.
Obviously, Blizzard isn't going to take this idea and run with it any time soon. They're far too conservative to consider something like this, if the past is any indication of the future. You can't really blame them, either -- it would be risky from a business standpoint. All of the things I've talked about are very much a "best case scenario" type of situation. I'd say we're still a few years off from them even tossing this idea around (if they ever do). Don't be expecting a magical cornucopia of MMOGs spilling into your lap any time soon. We're stuck with the current business model, for now. Still, it would be really great if companies would be willing to take a look at the past as we move into the future, wouldn't it?
It's sure nice to dream about what could happen.