If property can be rightfully inherited, either the descendants of whatever Pharaoh/other ancient Egyptian or the successor state of ancient Egypt are the rightful owners of what was taken from them. If your property may end up in a foreign museum millennia after you die, why even bother working?
Your argument relies on a number of premises that are not necessarily true.
First of all "Rightful inheritance" is a matter of human law not natural law. Whatever state or laws existed surrounding inheritance clearly do not exist anymore for artifacts more than a few hundred years old. It is absurd to say property rights of states existing thousands of years ago apply today. You won't find any courts arguing that. Absurd.
Second, descendants of millennia old property owners are both impossible, and too easy to find. Because of the way human genetics works you yourself may be a descendant. I'm a descendant of 13th century nobility, does that mean I and the millions of others with this ancestry inherit their various artifacts such as a cup or textile? Absurd.
>If your property may end up in a foreign museum millennia after you die, why even bother working?
I don't know. You'd have to decide that for yourself. To me the question is absurd. When I die I may will what is left of my estate, but I don't absurdly believe that thousands of years later my will be respected. Totally ridiculous.
These artifacts belong to the world. Any attempts by states to force other states or organizations to "return stolen items" is a mealy mouthed way of saying they want the value they perceive they have lost. This is absurd greed. Just like conquering land, the losers have no right to their lost land. That is never the case. And you will never see these same states trying to return the land they have "stolen" to the descendants thousands of years later, now spread myriad around the world, because it's fucking absurd.
When people die and a lot of time passes things become just things. Some novel state that has no true connection to, and in fact had no knowledge of, some past state has no right to property of millennia-dead people of that state. It's ridiculous.
If this was about doing the right thing then one would recognize that these are artifacts of our shared human heritage, and as such they should be kept in trust for the benefit for all humanity unbound to any particular regime cultural or legal. Of course it isn't about doing the right thing it's about stupid politics.
Dubious opinion
>Rightful owners
All long dead