Unfortunately sustainable is somewhat equivalent to money. Whatever work you do, and even if you love it, in general it needs to have a functional business model. Businesses that can financially support the people who provide them, tend to continue.
Personally, I believe this is the fundamental problem with many of the things that we now fondly think of as "old". Google groups? What was the business model? Did it make money? How could you make money from doing something like that?
The fundamental business model IRL used to be "fee for service". Not lock in. Not subscription. It works, because if people want the service they can pay for it. Okay, so hint: what are the issues of implementing fee-for-service on the internet?
hint number 2: someone mentioned banner ads in a comment. Is that fee for service? If not, for extra credit, what would be the side effects of a banner ad type business model? Are there useful services that could be provided with an alternative business model. Etc.