Jester David
Hero
I really don't why why they can't.I sympathize with the problem that WotC can't just release an "almost done" free version of the game and then expect everyone to purchase the final version.
WotC's audience is generally intelligent and tech savy. If they want something and don't want to pay there are, *ahem*, avenues they can take to acquire the media.
As such, the trick is not making it so people have to pay but so that people want to pay. Make the product so good that they want it as a physical product. High quality. Glowing reviews. Solid rules. Those will sell books.
It's much easier to flip through a book that a PDF, so long as it is well organized with a good index. The convenience alone should sell the product IF people want to play for more than a one-shot.
And if gamers have tested the crap out of the rules and made the edition better, that will help.
Look at Paizo and the PRD. They give away all their rules. And yet the books still sell. And Kickstarter. If gamers like a product they will pay for it in advance before it is even finished.
It wouldn't be hard. So long as they expand the flavour and make as many revisions as possible between playtest and final document. People bought 3.5e after 3.0e for that reason: the rules were cleaner. If there were as many changes between the free final playtest doc and the actual published rules people would buy it again. And they'd feel happier knowing they were buying a finalized product rather than something that would be rewritten in three years (like 3e) or continually revised with revised with online updates making the physical product useless (like 4e).