Ninshuu was abandoned because it was proven that, despite Hagoromo being someone that desired peace, HUMANITY was not ready for ninshuu. People will find reasons to war, and as the world has already gone through many thousands of years of being a ninja world, it cannot suddenly become a ninshuu world. Why would the people of the world decide to throw away their ability to mold chakra in themselves (the only way to weaponize it) for the sake of being able to connect to people around them?
Hagoromo chose Naruto because he was the best chance of peace out of everyone else there. He was not trying to bring back ninshuu, but rather chose Naruto who uses ninjutsu but happens to be an advocate of peace. Ninshuu is not possible at this point, so a ninja that whose words and actions achieved the same results. Naruto united the world, not only with his chakra (represented by giving the entire army his chakra cloak) but with his character. That is the best Hagoromo had to work with. There was nothing wrong with not bringing back ninshuu in my opinion.
What do you think?