This doesn't make sense at all, Sasuke still has the Rinnegan and can teleport to Kaguya's dimensions like a RinneSharingan user, his eye power increased since the time Hagoromo gave him his chakra.
Likewise Naruto still has Hagoromo's chakra because it grants the ability to fly which he did in the movies. Also the fact that he's meant to be the equal of someone with the tomoe Rinnegan further proves he still has Six paths Senjutsu.
He doesn't use RSM for the same reason Sasuke doesn't use the 6 Paths, there would be no plot for Boruto arc and Ootsutsuki arc because their abilities are just too strong. Anyone below JJ Madara's level can be dealt with using BSM and EMS feats.
The chakra Hagoromo gave them is permanent, just like the chakra he gave the first shinobi warriors that developed ninjutsu.