I can’t remember the plot of the Prince Caspian book at all, but according to Ross the film version departed significantly from the book. So I can’t say whether or not this objection applies to the book as well, but when I walked out of the theater I found myself badly disappointed by Aslan’s proposed response to conflict between the Telmurines and the Old Narnians. Offering to transport Telmurines back to the island their ancestors came from in the distant past would make about as much sense as rectifying the unjust dispossession of the Native Americans by suggesting that present-day Americans all go back to the countries our ancestors immigrated from.
It doesn’t make sense on a practical level (the Telmarines new neighbors aren’t going to be happy with it at all) and it doesn’t make sense on a moral level — as best I can tell, your typical modern-day Telmarine (as opposed to the king and a small circle of high officials) hasn’t done anything wrong. Putting this proposal in context of Prince Caspian riding to power at the head of an army of mythological creatures is just going to turn the Prince into a Quisling figure in the eyes of the human population.