The context is 150 years ago, NOT NOW.
I agree completely. It would be more difficult to justify a return to slavery now.
Also, I suspect that the transition to emancipation would have proceeded more quickly had it not been for the many Northern politicians who quietly but effectively put the brakes on any gradual emancipation because they knew that an end to slavery would result in many new House seats for Southern politicians. So long as the 3/5 rule existed, the South was cheated out of its proper proportion of seats in the House of Representatives.
But, you're right. I would not now support a return to slavery, with or without the 3/5 rule. Times have changed.