Naruto didn't lose Six Paths Sage Mode. He simply chose not to use it, because it wasn't needed. He was powerfull enough with Kurama's chakra. Also, Sasuke's left eye became a Rinnegan after he received half of Hagoromo's chakra. He doesn't have Hashirama's cells in him, Kabuto only used his studies of Hashi's cells to heal Sasuke. So Sasuke didn't have the requirments to awaken the Rinnegan on his own, and even if he had remember that Madara awakend his Rinnegan when he was near his natural death.
So both Naruto and Sasuke haven't used their own power in a long time, Sasuke more so than Naruto, because Naruto at first didn't want to use Kurama's chakra, but Sasuke used the cursed mark willingly. Naruto started using other sources of power after the Pain Arc. Sasuke had the cursed mark, and even when Itachi got rid of it remember that Orochimaru enhanced Sasuke's body with chemicals invented by him. So Sasuke after getting the cursed mark never was at "base form". Then he got Itachi's eyes and got the EMS, then Hagoromo gave him half his chakra wich in turn made his left eye a Rinnegan.