Add other perks to DDI that people want to pay for. Online character storage in the cloud accessible from a multitude of devices. There are any number of things they could do to steer people towards DDI with enhanced perks while releasing OGL.
I agree.
Paizo releases the vast bulk of their material as open content under the OGL. The only stuff that isn't open content is the fluff. They are doing great and they don't even have a DI web app.
One of the successes from Pathfinder is that it's easy to get into the game. There are at least a dozen people I've personally hooked onto Pathfinder because access to the rules was free. Eventually all of these people bought the core book and many went on to buy heaps of material, despite the fact that it's all free.
WotC could likewise just release all of their rule material as open content. What they need to do though is recognize that if they want to become a $50 million a year product then they need to transform themselves into more of a software product than a traditional print product. The end goal is to make a DI product that people will flock to because it's where everything is at in D&D.
One component in that would be to make it easy for 3pp to contribute within the DI ecosystem. Just like Apple has iTunes, WotC makes a digital environment where other people can prosper and once they are invested within it the market keeps shifting until it sustains itself under it's own gravity. Just like having an iPad or iPhone with all of those apps, a D&D DI would be the package that WotC owns where almost everyone goes to because that's where all the action is.
In defense, they have all of the rules as open content, and people can use them outside of the DI and do what they like with them, but WotC, with it's deep pockets, has already created an ecosystem that sucks everything towards it. While competitors have the material, they don't have the capital to really compete with what DI offers, but since that is a behind the scenes issue that fans aren't really paying attention to, it won't stir up bad PR. Fans get it however they want, either free or with all the trimmings, and adore WotC because they are being so "open."
In that kind of digital ecosystem, it could even draw in other established brands. They end up dominating the market and soaking up lots of money in a variety of ways because they have created an ecosystem that bearhugs anything that gets close to it.
In order to get to $50 million a year they would need around 400,000 subscribers at around $10 a month, or adjust those numbers down in various ways depending on how much revenue they can get from other sources, such as micropayments, books, license fees, etc.
Are they going to do this? I doubt it, but that's how it could be done.