It would be interesting to see what happens if players convert all their monks into cavalry, and then the Elves recruit them as blood kin.
The cavalry would essentially consist of two units (the player and the monks).
Then, by recruiting one unit as blood kin, the player would effectively eliminate two units at once (both the cavalry and the monks).
I think this is a strong argument for the Elves against retraining humans. After all, there are still two blood kin slots available, one of which could be archers, who also partially include peasants, and the other could be armored units, which may include retrained archers.