Excellent point.
Compare for example to the Scandinavian mercenaries in Russia who either (early on) 1. settled down, took on Slavic names and became basically your average hereditary gentry or (later on) 2. did none of the above things and were disliked by the population and sometimes targeted by popular revolts.
Same process happened with the Black Hats and Cumans, who, as allies settled within the nominal borders of the various principalities, became an important force in Kievan and Vladimir politics.
In any case, granting foreign (Lithuanian, Tatar, Circassian, but also others - Prussian, Swedish, Greek) warriors and for personal service on a hereditary basis is exactly the solution Russian princes came up with - in the very late 14th c. onwards, around the same time that a similar system developed in the Ottoman Empire. So it's not out of the question, but it happened much later OTL.