Slave revolts have a very difficult time winning. I think the Haitian revolt was the only one to ever succeed, and it had the benefit of being hundreds of miles away physically from its controller when it was busy fighting other powers. It is very doubtful that the Zanj rebels will be able to establish their own state.
The revolt occurred at a time when Abbasid power was already on the decline. A longer revolt would accelerate that decline. So even though the Zanj may not be able to establish a Basra sultanate, we could see earlier de facto emirates or even de jure sultanates on the periphery.
Mesopotamia is very much a "single unit" that promotes regional unification. An independent Basra of any kind is unlikely for long. Either Basra takes over the entire Mesopotamia, or Baghdad will.
I only see the Zanj rebels winning if another threat to the Abbasids crushes them creating a power vacumn that allows the Zanj to take control locally. If the Zanj turns themselves from a specific "slave revolt" into a more general revolt that could win support then it has a better chance to survive. There was still a large Kharijite movement at the time as well as Shi'ites unhappy with the Abbasids.