The waterfall model is obviously targeted at commercial projects, and even there it doesn't do terribly well. It fails horribly when requirements change – and that happens all the time, in both the commercial and Open Source worlds.

The incremental model is better: building an Open Source project one step at a time (first the editor, then the newsreader, ...) is a good start.

But it still doesn't address long-term lifecycle issues in a useful way. "Version 2.0" is just another run through the same cycle.

Let's look at some thoughts from the Open Source world.