Nonsense. If that particular plot of land was stolen then the people it was stolen from are welcome to assert their claim. If they can prove that they have one, that is. A thief ("conqueror") does not through their theft obtain the rights to the property which is stolen, and cannot rightfully sell what they do not own. The buyer acquires nothing, and (if innocent) is welcome to sue the thief for fraud to recover what they paid.
However, leaving the ownership uncontested for this long would certainly dilute any claim based on ancestral rights. If there comes a point where no one else is claiming rightful ownership of the land then any prior claim can be considered abandoned, and whoever happens to be using it at that point holds the rights to it by default. Even if it was originally stolen. So the question becomes, has someone maintained their claim to the land and passed it down to their descendents from 1066 until now? Or was that claim permitted to lapse?
The duke of westminster doesn't use the land, though, he rents it out. If use of land begets ownership, why does the claim originally stem from william the conqueror murdering a bunch of peasants rather than me renting a flat since 2005?
This idea of yours makes no sense.
Or inherit. Mayfair is rented out by the Duke of Westminster not because him or any or his ancestors did anything as crass as buying it but because he traces his lineage back to a mate of William the conqueror (note : not William the buyer. Conqueror. The distinction is important you see) in 1066.
Broadly speaking, all land wealth is derived this way, no matter how many hands it passes through on the way to its current owner.
You no doubt believe that the act of buying and selling land and working with it somehow "morally sterilizes" it. It does not. If Mayfair was sold it would still be conquered wealth.
I am very well aware that investments do not necessarily pay off immediately and you know very well that that is not what I am talking about at all.