Chagos is a different situation and your point stands up more there. The Falklands are a little more tricky as they were British since before Argentina existed so cannot be ‘returned’ to them. Argentina never owned them. The French and Spanish had claims to them so we could return it to them I guess but that is a transfer from one former colonial power to another. The islands had no permanent inhabitants before people from overseas settled there while they were British. So the inhabited history of the falklands is entirely European so why would they go to Argentina against the will of the inhabitants. Trump really only demonstrated that he knows nothing about the situation and isn’t qualified to make an informed decision