“which would have explicitly protected slavery in the US Constitution effectively forever.”
Not really. All the Corwin Amendment did was to prevent the U.S. Government from taking any action against slavery in any state where it was already legal. States were free to end the institution if the so chose to do so, and Congress had the authority to prevent the introduction of slavery into any territory they so chose.
Yes really. Slavery would be explicitly protected in the US Constitution and since 15 states still allowed slavery and it takes a 3/4 majority to pass a new amendment, it would be impossible to ever ban slavery without the consent of the 15 states that still allowed it. Everybody understood the numbers back then. It was ironclad protection of slavery - and Lincoln even said he would offer strengthened fugitive slave laws. Obviously that wasn't what even the original 7 seceding states were interested in because they turned the offer down.