Sasuke is already Kage level.
He kept training three years with Orochimaru while Naruto is doing nothing with Jiraiya.
MS only ruined Sasuke's style.
Sasuke didn't had a "teacher" to his MS like Naruto had for everything.
And actually Sasuke has to train to evolve his Susano'o and MS in real fights during Kage Summit.
At the beginning of Kage Arc against Raikage. Sasuke only Had Susanoo rib cages.
At the end of the Arc Sasuke dominate the Complete Susano'o but when he finally dominate his MS he is blinded
I see what you are saying but keep in mind the theme:
Sasuke is the natrually talented one like Indra. So his training mostly came actual battle
Naruto was the "Loser" who, through compassion and comradery, gained equal amount of power.
Neither one is better or worse than the other. Just 2 different ways of learning. It's like those people who can learn something from just reading about it while others need hands on lesson to learn.
Yeah MS is the true power of an Uchiha . They gain power in exchange lose part of their sanity. In other words they were born with that potential. Sasuke had to fight Itachi before he died.... How is that not considered work? :|
KCM or SM don't give Naruto any pain do they?
Sasuke on the other hand when using Susano'o in MS had to constantly endure a intense pain emanating from every cell in his body in addition to extreme fatigue. Sasuke had to endure these factors when facing the five Akame as well as Danzo
again true but not entirely. While yes MS you risk going blind after extensive and repeated use. Sage Mode comes with it's risk as well. Naruto still runs the risk of turning into a Frog Statue if he does not maintain a precise balance Nature chakra and his own.
So no he doesn't just get to use Sage mode for free. Sasuke's EMS however erases all of the MS weaknesses much Like there is no repercussion for Naruto KCM
I think Kishi was careful in this aspect to make sure the powerups of the rivals didn't eclipse one or the other.