Why would the domain scale higher?
? why would a non maximum ct technique scale above apex jujutsu. both attacks are straight forward in that their main offense is damage. (Fire arrow and Cleave and dismantle) and domain expansion puts these latter two at the peak. that's how the increase gets.
EXPANSION (展開 - tenkai)
The culmination of a jujutsu battle by expanding a domain.
Domain expansion… an innate domain imbued with a cursed technique, it is the act of embodying with cursed energy. After deployment, the cursed technique will have a sure-hit effect, and the user’s attributes also increase. It is called the “culmination of a jujutsu battle” because it can be said that a domain expansion = the end.
However, it consumes an enormous amount of cursed energy, so there are some disadvantages such as difficulty in using a technique after deployment.
its also why domain expansions easily shatter Simple Domains, with the reasoning being their output being too strong, too high. there's a hierarchy of ranking for these abilities in jujutsu level. it goes
Domain Expansion > Maximum CT > Any other ability in one's arsenal that isn't considered to be at such level of jujutsu.
MAXIMUM (極ノ番 - goku no ban)
Sorcerers fundamentally only have one type of innate technique. However, every technique can be expanded on through creativity, giving sorcerers their own original moves. With inherited techniques, sorcerers of the past pass on the original moves they create. Within all that, barring Domain Expansion, every technique has an ultimate move called the Maximum. Additionally, moves that are not Maximum that use the maximum output of curse energy are called Stack (載 - sai).
then there's this
THE INNATE DOMAIN (生得領域 - shōtoku ryōiki) ESTABLISHES THE FOUNDATION OF A TECHNIQUE — The "innate domain" is the space that we are born with, which can be called a "mental landscape" that embodies our inner heart. This includes not only sorcerers, but also non-sorcerers.
which all in all means your ct is way above any other jujutsu ability in your arsenal including maximum. or maximum output. as long as its in your own "home" (innate domain) which is like the prime environment for your techniques
the reason why i think domain > fire arrow is only in specific related to the damage they cause, not fire or temperature as cleave dismantle don't have these attributes but fire arrow and cleave&dismantle have a common main attribute is that they cause damage through the power they have. the second reasoning is that Gojo's first domain battle where he had his infinity down, or his second domain battle where he also had his infinity down, did not prompt Sukuna to ever pull up the fire arrow on gojo when he could and had the opportunity to do so, more so in the second domain battle where gojo lost. This is the gojo that he's trying to kill and is blatantly trying to do so. And he very well can use it when it is imbued into his domain.
[SUKUNA’S TECHNIQUE
Released by the King of Curses himself, it’s a slashing hellfire of slaughter.
Cursed techniques commonly only have one characteristic, but Sukuna’s technique has been confirmed to have at least two - slashing and flames - which makes it exceptionally powerful. Once it’s activated, it will destroy the enemy in an instant, tearing them apart and burning them down to their bone marrow.
解 - Kai (Dismantle), 捌 - Hachi (Cleave), ■ (?)
Sukuna’s slashing technique is Dismantle, which he uses normally, and the other is Cleave, which is able to be adjusted to the amount of cursed energy and strength of his opponent. When he releases the flames, Sukuna says “Open” (fugue)
Panel caption: It turns out that the three types of attacks have their own uses. Sukuna combines them depending on the situation in order to cleverly beat his opponent.]
Gojo and the gang already knows about the flames too, (Kusakabe has no reason to not tell Gojo or the gang) yet Sukuna never used it, ever and took the riskier approach against Gojo... because he already tried to kill Gojo with his domain and it didn't work, so his only way is to get the adaptation going. If the fire arrow was indeed the one shot move which many think here, then he would have used it with no doubt. literally no reason for him to not do it. especially when it is domain amped now too. and it isn't a separate ct or whatever but part of his ct.
TLDR: if sukuna never used fire arrow despite having the opportunity and ability to do so then it's pretty telling that it ain't gonna kill gojo or be the deciding factor to his win, or be enough to incapacitate gojo for a moment in order to then kill him. which makes it consistent with the fact that fire arrow is below peak jujutsu (DE) as it should be.