It isn't a plothole at all.
We've seen transmigration already. Indra did it to Sasuke / Madara, Ashura did it to Naruto / Hashirama. Sage of the Six Paths recently did it to both Sasuke and Naruto. Someone who has Uchiha, Senju, and leftover Juubi chakra who also happens to possess what is easily the most broken S/T jutsu that has canonically hijacked other dimensions can believably fill the necessary requirements for transmigration.
Throwing around the term "borrowed powers" like an insult is stupid. People throw that term around like it's a bad thing. Kakashi worked his ass off for that left Sharingan. As a non-Uchiha, he was constantly dealing with issues an Uchiha wouldn't have to deal with, like chakra draining. He got that Sharingan at age 12 and possessed it for 18 years; that Sharingan is more his than it is Obito's. The fact it was thrust upon him at a young age meant he centered his fighting style around it.
It's ridiculous that you people expect him to do something spectacular when he had his Sharingan for his entire life. Take away Sasuke's Sharingan, Itachi's Sharingan, or Nagato's Rinnegan and see where they'd be; they'd suffer worse than Kakashi. Take away Naruto's Bijuu or Bee's Bijuu, and while he'll still have Sage Mode, the drop in power from "God-tier" to Kage level is still huge.
We've seen transmigration already. Indra did it to Sasuke / Madara, Ashura did it to Naruto / Hashirama. Sage of the Six Paths recently did it to both Sasuke and Naruto. Someone who has Uchiha, Senju, and leftover Juubi chakra who also happens to possess what is easily the most broken S/T jutsu that has canonically hijacked other dimensions can believably fill the necessary requirements for transmigration.
Throwing around the term "borrowed powers" like an insult is stupid. People throw that term around like it's a bad thing. Kakashi worked his ass off for that left Sharingan. As a non-Uchiha, he was constantly dealing with issues an Uchiha wouldn't have to deal with, like chakra draining. He got that Sharingan at age 12 and possessed it for 18 years; that Sharingan is more his than it is Obito's. The fact it was thrust upon him at a young age meant he centered his fighting style around it.
It's ridiculous that you people expect him to do something spectacular when he had his Sharingan for his entire life. Take away Sasuke's Sharingan, Itachi's Sharingan, or Nagato's Rinnegan and see where they'd be; they'd suffer worse than Kakashi. Take away Naruto's Bijuu or Bee's Bijuu, and while he'll still have Sage Mode, the drop in power from "God-tier" to Kage level is still huge.