It is. But I don't think there is an alternative way to make it sustainable. There are just not that many people who are serious about self-education, and you won't like it to cater to the less dedicated customers.

> I can't help but feel that the people who would benefit from it the most are also the people least likely to be able to afford it.

Even if it were $10/mo, the people who would benefit from it the most (around the world) still can't afford it.