I didn't say it's not sustainable, I'm saying it's potentially not sustainable. Sustainability of tracking-free ads would be a good conversation to have.
I believe it can be pretty good. I go to goodreads to track new releases and yeah, I wouldn’t mind ads if it were non invasive. Everyone that goes to goodreads is interested in reading and books in some form, so why not show ads about books here, instead of when I’m looking for music gear?