> amount has been the same for centuries, and it can never be changed for any reason

That's only true in a narrow and a relatively obtuse way. For starters that varied to a huge degree between regions and types of contracts.

e.g. in England freeholds were indeterminate and or more or less worked the way you are saying.

However most peasants didn't have those, before the plague the overwhelming majority of peasants were villeins (i.e. serfs), inheritance was customary and lords were not legally obliged to pass it to the serf's descendants (also there were all kinds of fees, fines and stuff besides the fact that they weren't legally free and there was no legal system to protect your rights).

Leaseholds and copyholds became much more common due to labour shortages after the plague. leaseholds were not inherited and market price based. Copyholds were inherited and rents customary fixes (but again lords could and would impose all kinds of arbitrary fees to get their cut).

Then you had the enclosures starting the 1400s (a lot of the land peasants relied on was common)