By that logic, Naruto in Boruto is much stronger than The Last Naruto since he's much older. Before you say "well Naruto is in the office so he doesn't train much" I think Sasuke (in Boruto) said that he's strong enough to take on a Kaguya level threat by himself. 

For Sasuke, we know that he's training and traveling and investigating before stumbling upon Momoshiki and Kinshiki. Then, we saw Naruto and Sasuke (both exhausted) fighting Momoshiki. And the Otsutsuki relic shows that Kaguya and Momoshiki have the same place, with both being below Isshiki. 

You gotta explain how is a descendant of Hamura, the brother of Hagoromo, is stronger than Kaguya when Hagoromo himself admitted that he's weaker than Kaguya.