This would still require customers, a significant portion of which they've likely now lost permanently.

Maybe, maybe not. People will probably still buy their completed games in the future, it's just going to be a fraction of total sales (I was referring to these sales when I mentioned residual income).

This may not even be possible as the licenses may not allow for sale to a third party.

Maybe IP was the wrong way to word it, but whatever the term is for ownership of the games. I think you are right that they would not be able to sell the licenses so that a third party can do anything new with them, but Telltale already acquired the licenses to use the properties for the games they've already released and I don't think it's possible to restrict that ownership from transferring.