Without importation of African Slaves, slavery would have eventually dwindled.
In theory you had not had any importation of African slaves for more than 50 years. Yet slavery grew steadily during that period. In fact, of course, there was a brisk trafficking in illegal slave imports, and demand for slaves regardless of source was undiminished. So what makes you think that would have ended simply because the confederate constitution said it was illegal?
Hogwash. The wealthy slave-breeders and traders in the south were in favor of bans on importation because they did not want competition for their business. they were, however, more than capable of meeting the demand for slaves in the CSA.