What makes you believe this? I don't think there's actually much evidence for it in the recent past (previous 300 years, say). Part of the reason that the British Empire was able to get rid of it with minimal fuss in the early 19th century was that it was no longer considered economically viable, and was already in a natural decline in many places.

Most of the resistance to the abolition of slavery in most places was social; it was an upheaval in the way things were done. "GDP will drop" was not, as far as I know, a major concern, nor did it generally happen.