They weren't though?
Hagaromo turned up in a spirit quest vision out of nowhere, explained that the thematic parallels of the Senju vs the Uchiha being embodied by Naruto and Sasuke weren't just theming but were actually the product of the constantly reincarnating spirits of his two children throughout history. Rather than being a statement on "People who do not learn the lessons of history are doomed to repeat it," Naruto as series ends up with "No, history is just the same conflict again and again because destiny," (which is pretty lame in my opinion)
He then asked both them "Hey, what do you think being a Hokage means?"
Naruto went "Bonds and comradeship," Sasuke went "Revolution and executing my vision of peace,"
Hagoromo said "Neat, have a powerup," and gave them both a big powerup.
They weren't earned by character development because neither character developed significantly between the revelation of the true nature of the historic conflicts in their world being some rando alien spirits getting revealed to them and them being rewarded with a powerup. It's incredibly generous to say "they got them as reward for character development," when said development boiled down to a quick conversation and then they got a powerup despite answering entirely different things.
To me, it's also worse on a larger scale because Harogomo's presence in the narrative and the whole destiny angle boils down the complex question of "how does one end a generations long period of war and lay ancient and potent grudges to rest," to the answer of "oh, you just have to beat this one guy because of destiny,"
As I've said in the past, I like Naruto and generally think it tries to be a lot more complex and interesting in its theming and storytelling than people give it credit for.
This aspect though? This and the whole Ohsutsuki plotline? Terrible, actively made a lot of things much less interesting.
We already had a compelling and complicated story of the most hated and demonised person in his society trying to rise to position where he could create meaningful bonds, be granted a sense of self worth and effect change to end wars, paralleled against one of the most lauded and exceptional people in said society descending into hatred, starting wars and cutting himself off from that society because he believes the exceptionalism nonsense he'd been fed since birth.
That's good and powerful stuff.
You don't need "And also they were the reincarnated spirits of some fucking guys we've never heard of," on top of that. That is a hat on a hat.
TL;DR - adding destiny to an already interesting story is a bad idea. destiny sucks unless it is an antagonistic narrative force.