I think it's rather ridiculous that so many are trying to offer black and white answers - "if blacks were not slaves then they'd still be in Africa and would not be a part of this great nation" <--- This is a bull**** response. especially since it apparently stems from the false notion that this country owes all it's greatness to white settlers. This countries greatness is nuanced and can be attributed to all the minorities that migrated here.
Especially the black slaves on who's backs much of this country was built upon. A lot of the economic success of the south relied on slaves.
So in an alternate universe where America never had black slaves the possible trajectories that alternate sequence of events could go is endless.