Jujutsu Kaisen introduced the most intriguing outliers through Toji Fushiguro and Maki Zenin. In a world built on Cursed Energy (CE), these two individuals were the exception. What set them apart from the rest was one thing - Heavenly Restriction. They were presented as Binding Vows placed on sorcerers at birth which affected their CE, giving them either immense limitation or physical capability.
In both Toji and Maki's case, they had very little to no CE but were given superhuman physical attributes. The latter was even more distinct, given that her sister's death was necessary for her to fully awaken her Heavenly Restriction. Once that happened, she had likely surpassed even Toji. But many have wondered the consequences of Maki dying over Mai, i.e., a riveting "what if".
Disclaimer: This article reflects the writer's opinion.
Jujutsu Kaisen: What would happen if Mai survived instead of Maki

As a brief refresher, Jujutsu Kaisen puts forth Maki's awakening during the Perfect Preparation Arc. She goes to the Zenin household to retreive her Cursed Tools to prepare for the battle ahead. But is compelled to fight her father after seeing a gravely injured Mai. While she does dominate, she ends up being defeated and both sisters are thrown into a pit of Cursed Spirits.
Barely alive, the sisters reconcile and Mai sacrifices herself, taking Maki's last bit of CE with her thereby unlocking her full Heavenly Restriction. After this, Maki becomes a different beast, compared to and even believed to have grown stronger than Toji Fushiguro himself. However, there has been curiosity over whether or not the same would happen with Mai if Maki had sacrificed herself.
Simply put, Mai would not get the same power-up as Maki. For one, Mai possesses very low CE reserves, i.e., more than a non-sorcerer but less than a human. Between the pair, she was the one with more CE and as the lore explains, them being twins means CE considered them a unit. This is why Mai's death took away whatever little CE Maki had. In other words, Mai was halting Maki's potential.

In a reversed case, if Maki had died, Mai would have likely received a small boost in CE. Usually, she could use Contruction to make only extra bullet. In other words, her Cursed Technique was inefficient given its capacity. If Maki died instead, she would probably be able to construct a few more bullets, i.e., complete the technique, but nothing more.
Again, Mai didn't have Heavenly Restriction, so it Maki's death would nothing for that. Even in terms of raw talent, Maki was miles ahead, as Mai didn't want to be a sorcerer. She has previously admitted to this and Jujutsu Kaisen doesn't paint her as someone with untapped potential. A powerful mindset breeds power and Mai only became a sorcerer out of compulsion, hinting that even this wouldn't work.
Ultimately, it cannot be denied that despite being one unit by CE, the twins were never equal, It more a case of Maki's brilliance being suppressed and chained by emotional and spiritual bonds. Contrastingly, Maki’s death would have maginally improved Mai. She would never have attained a similar level of power as her sister. Not power, their arc presented the cost of severing their shared identity.
Final Thoughts

All in all, Jujutsu Kaisen kept Maki alive for good reason. She had narrative significance and even went toe-to-toe with the Demon King. The latter was intriguied by her mere existence, labelling her as one who voided the idea of Jujutsu sorcery. True enough, she was the exception in a society driven by CE and the flashy techniques it could be channeled into.
In this explored "what if", Maki dying instead of Mai wouldn't have yielded a similar result. Mai's reluctance to be a sorcerer, no latent potential and an inefficient Cursed Technique together hold her back from achieving what Maki did. Also, Maki's little CE would only afford Mai's CE reserves to become comparable to a regular sorcerer and not exceedingly large, akin to other socerers in the series.
Related links:
- Maki may not have needed Mai's death to power up in Jujutsu Kaisen
- Why did Maki slaughter the Zenin clan in Jujutsu Kaisen? Explained
- I thought Mai was Jujutsu Kaisen's weakest character until I realized what her Cursed Technique Reversal would be