For ordinary Uchiha, evolving the Mangekyo into the Eternal Mangekyo requires transplanting another pair of Mangekyo from a direct blood relative.
In Shiro Gen's view, that is only because most Uchiha lack the required mental strength.
If the mind is strong enough, that condition can be ignored.
Consider their progenitor: Indra, elder son of the Sage of Six Paths, heir to the Sage's eyes.
His Sharingan must have been Eternal Mangekyo, not a Mangekyo that burns out with use.
Of course, the best proof for this theory is Shiro Gen's own entry talent, which reveals how to upgrade the purple entry [Mangekyo Sharingan (Purple)]:
Method 1: Attain [Yin Release Sovereign (Gold)].
Method 2: Transplant a Mangekyo from a direct blood relative.
Method 3: Possess Senju blood of sufficient purity.
Shiro Gen had no direct kin to meet Method 2. As for Method 3, he could in theory obtain Tsunade's blood to gain Senju purity. However, Method 1 was by far the simplest for him.
"Until [Yin Release Dominion (Purple)] rises to [Yin Release Sovereign (Gold)], I should use Mangekyo arts sparingly, if at all."
"Losing vision is not a pleasant feeling."
"Next,"
"I will devote myself to developing Flame Release."
...
...
A month earlier.
Shimura Danzo stepped into the Hokage's office to face Sarutobi Hiruzen.
"Monkey, there is movement in the Uchiha."
"Last night, nearly all their jonin returned to the compound."
"I do not know what happened, but afterward they changed. They look feverish, filled with hope."
"We must investigate."
Hiruzen drew on his pipe in silence.
Exhaling, he answered evenly:
"I know."
"But the Uchiha are part of the village."
"At the borders, frictions with Sand and Cloud are worsening."
"War is near. We cannot move on the Uchiha now."
Danzo's expression cooled.
"Monkey, sometimes internal peril is worse than external."
"I can provoke Cloud and Stone to occupy them."
"I want authorization to investigate the Uchiha."
Hiruzen's eyes brightened, then he shook his head.
"No. Not now."
"When war breaks, the Mist will surely join."
"Then we can send the Uchiha to face Mist."
"That will both ease investigation and wear down their strength."
Danzo fell quiet. He disliked waiting, but the plan had logic.
"Fine."
"This time, I follow your lead."
Relief touched Hiruzen's face. Then a thought struck him.
"What of Hatake Sakumo?"
Danzo pondered, then replied:
"Likely no problem."
"He has stayed indoors, waiting for the storm to pass."
"A clever choice."
Though his words praised Sakumo's prudence, pride edged Danzo's tone.
Hiruzen knew him well and understood the rest without being told. Even a clever man would be bound by Danzo's net. Sakumo would remain a blade in the village's hand, never a Hokage, never a power broker.
Hiruzen disliked Danzo's methods, yet the outcome suited him. Still, Sakumo was the famed White Fang. To be safe and to keep the blade close, Hiruzen had his own thoughts.
Sakumo's son was Kakashi.
Perhaps he could study under Minato.
That way,
Sakumo would never defect.
Such calculations Hiruzen would not share with Danzo.
"I see."
"Hmph."
Danzo snorted, unimpressed by Hiruzen's feigned concern.
The plan was yours, the result is what you wanted, and now you pretend to worry about Sakumo?
Hypocrite.
Such a man should not be Hokage.
I am the one most suited.
He said none of that aloud.
"If there is nothing else, I will go."
"Nothing more. See to it."
"Mm."
Danzo left. Hiruzen smoked and thought.
If Kakashi studies under Minato,
who should teach Shiro Gen?
Yuuhi Shinku?
...
...
Flame Release Chakra Mode.
At Shiro Gen's thought, black flame of Flame Release flowed across his skin,
condensing under his command into a regal, forceful armor - an energy mantle blazing and grand.
"This really is stylish."
"And Flame Release is not Fire Release. It has traits Fire does not - free shaping, free motion, a self-moving glide."
"With these traits, I do not need seals. I can simply..."
He directed the Flame chakra and formed blades.
One sword in his hand.
Nine swords floating at his back.
Under his control, the nine Flame blades flew like swords of legend.
They spun and circled.
"It truly feels like sword-riding."
"And now I can command nine Flame blades with ease - Flame flying swords."
"Also,"
"the Flame-Sword Phantom Body is much stronger."
He gestured.
The nine flying blades shot into him and merged with the Armor of Flame.
"As for Shiro Gen-ryu, its power has doubled."