This is why grants are really important. That usually means deliverables in a specific timeframe. To me, that elevates open source from a full-time hobby to a job.
Other kinds of income are also good ways to fund open source like service, consultancy, support and even paid open source apps (which works particularly well for apps that have enterprise oriented features, turns out it doesn't matter that the source code is available under a free software license if it's convenient enough to click and buy).
Coincidentally, this is how I get paid :-)
Still, grants should not be ignored indeed.