So to properly answer this question we have to examine the timeline of the Egyptian-Kushite relationship.

The earliest iteration of the Kushite Kingdom was the Kingdom of Kerma which came about somewhere before 2500 BC and it differed from Egypt in that it was a highly centralised Kingdom and differed from the Egyptians culturally except for the religion,it was known that Kerman Kings worshipped the god Amun but he was the only God that could be worshipped in Kerman territory unlike in Egypt which was more polytheistic as you know,this Ammunic pseudo-monotheism is something we’ll continue to see with Kush as the millennia go by.

But it’s also important to mention the Naqada culture which was present in Northern Sudan and Southern Egypt and clearly influenced the formation of Ancient Egypt and Kerma too.

The Kingdom of Kerma continues to grow in power and annexes nearby Nubian kingdoms and subsumes their territories and at this point the Egyptians realise that Kerma is a problem and we know this as we see increased fortifications being built in the southern parts of Egypt in order to protect from Kerman raids and to secure the trade.At one point the Kermans and the Hyksos ally with each other in order to destroy Egypt and they divide it up between themselves with the Hyksos taking Lower Egypt and the Kermans taking Upper Egypt and they whittled away at the Egyptians territory until the Pharoahs only had command over Thebes and the immediate area around it,we almost never learned of these events as the Ancient Egyptians found it very humiliated and went to great lengths to expunge records of this and one of the only reasons we know this happened is because while the Kermans were occupying Upper Egypt they took many Egyptian artefacts as treasure for the King of Kerma,one of the most famous examples of this is the statue of Lady Sennuwy which was part of the treasure of some local Kerman governor and was excavated from Sudan.

Now,the Kermans had chosen not to completely annex their half of Egypt and integrate it into their territory and this would be their critical mistake as the Pharoah Ahmose and his successors would campaign to expel the Hyksos from Lower Egypt and they were successful in this and when finished they would turn to Kerma and it’s occupation of Upper Egypt and due to the military skill of Ahmose and his successors and due to barbarian invasions from the peripheries of Kerma which had crippled it,the Kingdom was in no position to repel the Egyptians and by around 1500 BC the Kingdom of Kerma was annexed into the Middle Kingdom of Egypt.

The Egyptians would go to great lengths in this time period to Egyptianise the Kushites and indeed the famous temple complex of Abu Simbel was built by the Egyptians as response to Kushite rebellions to show the power and majesty of Egypt to the Kushites and how Egypt would eternally rule in Kush.Ironically,not even two centuries after the Abu Simbel complex was built the Egyptians lost control of Kush and the resultant Kingdom of Kush was born somewhere around 1100 BC with a more Egyptianized culture although in my opinion it was superficial adoption of Egyptian aesthetics more than anything,the qualities of the native culture still remained like highly centralised government,gender equality,the Kushite system of succession etc

When you talk about “we’re last protectors of the Ancient traditions since the fall of Egypt” you’re referring to the narrative that Kushite rulers told during the Napatan period as they conquered Egypt.

The conquest of Egypt began like this,the Kushite King Kashta peacefully brought upper Egypt into his dominion by installing his daughter Amenirdis I as the Divine Adoratrice of Amun(head priestess of Amun) although it is recorded that the Kushites had already installed their garrisons at Thebes anyway so as to prevent incursions from Upper Egypt.

Then Kashta was succeeded by King Piye who completed the conquest of Egypt via military means this time.Now,Piye was very religious and a dedicated worshipper of the god Amun and used this as an excuse to conquer the remainder of Egypt,framing it as a holy war against heathen Lower Egyptians and as a restoration of the proper worship of the god Amun,all this is recorded in the Victory Stele of Piye which recorded how the conquest of Egypt unfolded.

It was recorded that this was a time of great prosperity for Egypt and Kush with more temples and pyramids being built than since the Middle Kingdom and many successful campaigns were undertaken against the Libyans (all Berber peoples were called Libyans by the Kushites and Egyptian sources) with records detailing the destruction of Libyan principalities and chieftaincies and the same occurred with campaigns in the Levant,the inhabitants of whom were all called “Asiatics”.

The Kushite Empire came into conflict with the Assyrian Empire and although the Kushites dealt the Assyrians one of the greatest defeats in their history and as famously recorded in the Bible saved the Jews from the wrath of the King of Assyria,after a back and forth between Kush and Assyria,the Assyrians expelled the Kushites from Egypt,although they did not attempt an invasion of the Kushite heartland proper.

Later generations of Kushite rulers continued with this narrative that Upper Egypt was the birthright of the Kings of Kush and this idea was reaffirmed by the fact that Egypt was now locked into a cycle of being conquered by one Empire and then being conquered by another and another etc with the religious traditions dying out.

Sources:

Török,László(1997). The kingdom of Kerma: handbook of the Napatan-Meriotic civilization, Part 1, Volume 31. Brill

A. Lobban Jr. Historical Dictionary of Ancient and Medieval Nubia. (Scarecrow P, 2003).

L. Haynes. Nubia: Ancient Kingdoms of Africa. (Acme Printing Company, 1992).

Hirst, K. K., 2015. Kerma – Ancient African Capital, Opponent to Egyptian Pharaohs.

Omer, I., 2004. The Kush Kingdom at Kerma.

Omer, I., 2009. Observations on the Deffufas of Kerma.