They won't be able to hold it for long, and it's likely to break off pretty soon. In the long term, though, a successful Persian conquest may cause Kush to become a bit more interventionist in the future. It may be prompted to help stir up rebellions in Egypt, just as the Greeks did, as a means of antagonizing the Persians. The Kushites may even once again try to conquer Egypt, again under the guise of being "liberators" and guardians of Egyptian culture and kingship, just as they had in 8th/7th centuries BCE.