Power has been borrowed from the beginning of the series if you really think about it...
Think in Part I, Zabuza.
His sword is an external element that was given to him by somebody or taken for himself (don't know how he actually got the sword or became a mist swordsmen) but take that sword away, and he's majorly powered down.
Naruto got the Kyuubi before Part I, and you would have had to have been a fool to think he wouldn't have been able to control that power even just five years ago reading the manga.
Gaara had borrowed power from Shukaku in Part I.
Kakashi got Obito's Sharingan revealed in Part I.
To some extent, summonings are a "borrowed power". It does take your own chakra as a medium to call forth whatever your summoning, but the summoning does not actually have to help you and can leave at any given moment. The summoning also does not have to do as you say and has it's own free will. Those were revealed in Part I.
Uchiha also have external elements to awaken their Sharingan. What if they never went through a life of tough experiences that opened up their Sharingan?
Naruto got the Kyuubi from his dad, and Sasuke got the EMS from Obito (since Obito saved Itachi's eyes and decided to implant them it counts, what if he decided to NOT implant them? Sasuke would be blind, but Naruto would still have Sage Mode which he earned by himself).
In Part II, Jiraiya was able to use Sage Mode because his summonings (which again, are free willed creatures, fully able to not help him) decided to use Amphibian Sage Technique to help replenish his Sage energy).
Danzo had TONS of borrowed power.
Kabuto is nothing BUT borrowed power.
What I said about the original Sharingan shouldn't be taken to heart, seeing as almost all the Uchiha were ninja, and being a ninja puts you in a life of loss that would trigger at least a basic Sharingan, But other than a select few, what does a basic Sharingan do for anyone anymore?
Evidence also somewhat suggests that Itachi activated his Mangekyo because of Shisui's death, but since Shisui planned his death, that in essence, gave Itachi the Mangekyo.
We could play the borrowed power game all day, but in the end, it's what makes the manga interesting.