Your question assumes that all people in the Ancient Egyptian state, the superpower of the time, were all cushites. This is far from the truth. Think of the present day superpower - the USA. Every known race and ethnic group is represented. In some cases some ethnic groups have formed enclaves where they exclude others to the extent of publishing newspapers in their ethnic languages. This was the case in Ancient Egypt. If you read Petrie's history of Ancient Egypt, you will also see that vassal states sent tribute annually to Egypt in the form of produce and 'men and women'. This means that the Cush, punt and Mezau which I believe had black, possibly Bantu populations sent their people into bondage in Egypt over many centuries. The states in Palestine, Libya and southern Europe also sent their people, making Egypt a cosmopolitan empire.
In my opinion, when migration had to take place, most people retraced their steps home, but of course they took with them friends and in-laws from other races.
I do not believe that there was a time that The Bamaasaba or even the Kikuyu were BANTUISED. They more likely absorbed the Cushites and Caucasians who chose to join to join them in the southern march.
Note also that the terms Bantu, Cushite and Nilote are of recent coinage by Greenberg and generally accepted by anthropologists as language classifications and not Race classifications.