You will put a sizable dev effort into a 5.99 IA purchase that statically will not sell nearly as much as the game itself. Maybe that's not the exact case but this is the balancing act.
Personally I'd like to see the brothers produce concrete milestones. For instance, in the first month you have to ship the first paid version. Or whatever. So far, they haven't put their butts on the line for anything less than a year.
Some things i really loved about the devs of BADLAND are that not only did they provide a huge amount of free content before the paid expansions, but the expansions were absolutely worth it! Further more they recognized the existence of a particular audience segment that would get absolutely hooked to their game and would aspire to be the "hard-core" ones. They rewarded them by providing challenges that would unlock the premium content. And while these were reasonably tough, they were also doable.