For majority OSS projects out there, that is not sustainable. As you said so yourself, when the money runs out the project is in trouble, because when those people inevitably get real jobs to cover their livelihood they become less active or might even leave, feeling disincentivised (sp?). And with them the knowledge about all the little obscure stuff goes away. Plus, if you had a successful project and a nice job you liked, would you quit it because some people on the internet promised to pay your salary? You just have no guarantee they will do that.