"If people were removed by genocide and ethnic cleansing..."

If people were removed by genocide and ethnic cleansing then it determines and should determine which legal principle modern law ought to adopt.

Here we need to distinguish facts from legal consequences.

The historical fact might be:

A population was forcibly expelled.

That historical finding can be extremely important. It may trigger legal rules under international law, such as rules concerning refugees, unlawful displacement, or property rights.

However, the historical fact does not itself contain the legal rule.

Rather, the legal rule comes from constitutions, statutes, treaties, or customary international law.

For example:

Historical fact: A village was destroyed and its inhabitants expelled.

Legal principle: People unlawfully displaced have certain rights of return, restitution, or compensation under applicable law.

History establishes the factual predicate; law determines the legal consequences. In many legal systems, the historical finding is sufficient to activate those legal consequences, but analytically the historical fact and the legal rule remain distinct.