“It was the movie that killed Disney traditional animation.”
Eh, that’s not quite true. Princess & the Frog did deal damage towards Disney traditional animation, but it didn’t really kill that medium. That “honor” goes to the 2010 Winnie the Pooh movie, the actual last Disney traditional animated feature film to ever be released. Though in Winnie the Pooh’s defense, the only reason it did so poorly was because it was stupidly released the exact same week as Harry Potter and the Deathly Hallows Part 2’s first week (which would never have ended well regardless of whether it was CGI or traditionally animated). Also, to be fair, P&TF also ended up released around the time Avatar was released and that movie being a box office hit (allegedly regarding that last bit, anyway).
John Lasseter, who was running Pixar and Disney animation at the time, thought it was the movie that killed traditional animation at Disney. A Winnie the Pooh show was never going to be a monster hit. Disney has different expectations for different movies. They considered "A Goofy Movie" successful with a total gross of $36 million, as it was based on a Disney sidekick character.
Princess and the Frog was supposed to be one of the tentpole franchises, and Tiana was supposed to be up there with Ariel, Snow White, etc.
Princess and the Frog didn't fail because it had a black lead. The story was convoluted, it didn't have an earworm song like "Let it go" or "Colors of the Wind," and there wasn't a strong sidekick character, although I kind of liked the firefly.
Disney had been dithering on traditional animation for a while, and Princess and the Frog was a test case to see if it was still viable.
Most of what is called traditional animation now is done on a computer, anyway.