It strikes me that if Growl were to disappear, there would be a chorus of users chiming in that they'd "happily pay for the software" as we usually see with popular free utilities that disappear. Here we have someone who finally decided they wanted to profit from their effort, and we see the flip side. A definitive lose-lose situation.
If you listen to the interview, Metzger actually seemed happy that there would be a paid version, thinking it meant that it would be maintained. Instead, the paid version didn't work for him and fixes didn't seem forthcoming. So he rolled his own and contributed. I'm not saying that your scenario wouldn't happen, but it doesn't apply in Metzger's case.
How? An open source project goes commercial and someone forks it to keep an open source derivative going. The system works. I don't see how this is a lose-lose. If the Growl dev loses users then it just goes to show that it's not worth paying for. I won't offer my opinion on whether it's worth $2, especially at this point in the project lifecycle, but you might be able to guess it.
I'm speaking from the perspective of community response. If the developer grows tired of maintaining the application and decides to abandon it, the community reacts negatively. If the developer grows tired of maintaining the application, but decides that being paid to do so is sufficient incentive, the community reacts negatively.
That's negative-negative, or lose-lose from a community response perspective.
