Not true. Slave based economies are generally stifled economies. In which you have a wealthy elite that live in luxury but are usually cash poor because it is expensive to maintain slaves. A large section of free but poor who have few job prospects because the upper class don't want to pay wages. And then the slave class which don't actually benefit the economy.

The rise of the middle class, professionalism and the need for skilled labor is what leads to the decline of slavery.