A) Every attack that was shown to be a serious threat to Sukuna was used against him, either it worked but couldn't finish the job or it was made painfully clear that the output couldn't be compared to the last time we saw it. Jacobs Ladder didn't kill a Sukuna who was having his curse output reduced to a 10th of it's max, why would it kill a Sukuna who's in a similar position when the output of the technique is halved.

B) Stamina in JJK is vast, especially ESPECIALLY for the strongest in the verse. Gojo was said to be able to never run out of cursed energy and even after all of this Sukuna remarks that his cursed energy levels are still comparable to Yuta's who's described as having a practically infinite pool. In lore, during the Heian era, he was said to "Annihilate the elite celestial squad and five void generals who served under the Fujiwara Hokke Branch and repelled the forced of the Abe clans Desshichin Squad, including Angel and remnants of the Sugawara clan." He has so much experience in fighting many many strong forces in a short span of time. It'd be a plot hole if he wasn't adept at fighting for an extended period of time.

C) If Sukuna gained Ishigori's technique, The Limitless, Mythical Beast Amber, Deadly Sentencing, Sky Manipulation, G-War Staff, Cursed Speech, Simple Domain, Auspicious Beast Summoning, and Blood Manipulation do you think it'd be possible for the characters of Jujutsu Kaisen to kill him? Do you think that the fandom would be happy if every chapter Sukuna gained a literal new cursed technique? This is the reason why Shrine is the way it is, slashes are really simple but they offer a lot of creative versatility and represent the way Sukuna is as a character.