First: Sleeping Beauty didn't have a re-release until 1971; after the death of Walt.
Second: The Black Cauldron didn't kill the company. The story of that movie is rather problematic and there are a lot of issues due to a lot of the upheaval at the time, particularly once Eisner and company arrived, saw this and tried to bury the film as if they wanted to get rid of it. Honestly the film is a bit of a victim since it feels like a lot of issues lead to it's failure and by the time the studio was beginning to maybe perhaps give it a chance (in the same way they ultimately even re-released Sleeping Beauty turning it over several re-releases from a bomb to the 2nd biggest grosser of 1959), The Little Mermaid and the Disney Renaissance happened and thus it was just buried and forgotten even further.
If Eilonwy was to ever be finally respected as a true Disney Princess (which I doubt will ever happen; heck the 35th anniversary is this month and the studio is ignoring it while acknowledging all the other 5/0 anniversary films, especially "Princess" ones like Cinderella and Pocahontas...and of course Tangled is going to be later this year), then her message is one of survival, both as a message for her dealings with the Horned King and ultimately the film's own situation.