I don’t think it is failed. It has reached self-sustaining levels for many topics. It will need further growth to make smaller, niche topics self-sustaining. Whether this growth will take place is an open question. I know my instance is growing in terms of activity, but I’m not sure how others are faring.
But as long as it isn’t shrinking, I think it’s well-positioned to absorb more growth as users discover it or become disillusioned with Reddit or other sites in the future.
Almost everyone.