There's a less valid argument here that this will all fall apart when people lose interest/capacity and therefore is it not sustainable.
On the first point, good. People should stop doing voluntary stuff they no longer have the resources or motivation to do, boundaries, rest and self care are good actually.
Secondly: the idea that since it could spontaneously stop any time, and probably will, that is inherently by definition unsustainable.
On the same token one might argue singing a song in a group is an unsustainable thing to do. Because you cannot sing the same song together forever without stopping.
You must rest when you're tired, then perhaps sing a different song, maybe with different people. This doesn't make singing songs an unsustainable practice. Most people in sustainability would agree singing songs in groups has a lot of offer sustainability from many angles.