Exodus 9:9 Then the Lord said unto Moses, Go in unto Pharaoh, and tell him, Thus saith the Lord God of the Hebrews, Let my people go, that they may serve me....
And why do you think they stopped building pyramids? No slaves to build them, not the lack of belief by the Pharoah's of what the pyramids represented....
***The theory that they could not have been slaves is based on three known facts, but all of those facts are subject to interpretations.
First, even high-ranking workers (found in tombs near the pyramids) have stress fractures in their bones, indicating heavy manual labor. The theory goes that the vast bulk of workers could not have been slaves if the overseers were working just as hard. Maybe, or maybe that was just what the job called for. Perhaps the high-ranking overseers worked just as hard, but they doubtless got paid and fed better. Did the lower-ranking workers get paid at all?
Second, there are hundreds of tombs of people buried in the shadow of the pyramid. Why would slaves be allowed such a privilege? Well, who says it was a privilege? Maybe the king wanted his servants nearby. Also, it is estimated that 25,000 people worked on the pyramid, but only a few hundred are buried there. Obviously, the lower-ranking workers (slaves?) were not given such burials.
Third, there is graffiti inside the pyramid referring to the "gang of Khufu" and other work gangs. Ok, why does the fact that the workers were organized into gangs mean they were not slaves? Why does the fact that at least one of them knew how to write mean that the other members of the "gang" were not illiterate peasants or slaves?
Can we not just say "we don't know" about this one? What is the actual definition of slavery? Well, other than the "being property of someone", a slave is an unpaid worker who is not free to leave and find other work. If you were a farmer in Egypt you only worked so many months a year (due to the Nile flood cycles) so during the off months you sat idle, and either ate food you had stored, or you starved. If the king said you could work on his pyramid for three meals and a place to sleep, maybe you really had no choice? If you had no choice, were you any better than a slave?
You could say you don't know, or you could wonder where the bones of all those slaves are, and reason with yourself why you don't believe our Historians, when today they are just having a guess!!