It wouldn't have enshrined slavery into the Constitution, it would have enshrined the right of each state to choose how to handle slavery. Practically speaking it would have ensured slavery in the short-term, but anti-slavery elements (particularly those who considered the eventual abolition of slavery to be Providential) could have held out hope that the individual slave states would eventually choose abolition.
Furthermore, the perceived choice at the time was between Southern secession and a compromise like the Corwin Amendment. The Corwin Amendment would not have freed any slaves, but neither would have secession. So it might be understandable why those against slavery might not have a strong opinion on the matter.
That is an interesting comment.