Not sure if I agree; sustainably can be different things for different people. For some it might be that the platform has a large number of users, but for Beehaw it is to have a safe community without bigots and trolls. It’s not sustainable for targeted individuals if they are met with hate speech, racism, transphobia etc every time they visit social media.

The Beehaw about pages explain their philosophy in more detail: https://beehaw.org/post/439918

The Beehaw admins have just been doing exactly what they said they would in their signup page and community guidelines, yet a large number of people seem to have gotten surprised that they kept their word.