@marcyvq
It's not instantaneous even under normal conditions; we don't know the extent of the effect caused by Power's, uh, power, but we've seen her delay Makima before when she escaped with Denji, and considering she's been successful at that, it's just that strong. Freezing and physically separating pieces of flesh should also have helped prevent or at least slow down the regeneration process further.
@Dranto
This ending has been planned all along; it's foreshadowed right on the
original cover, and the pieces have been set up well in advance. We can see Denji mingle with the spare bodies while the attention is on Pochita/Chainsaw Man. We were shown the capability to regenerate the entire body around the heart, as well as Chainsaw Man's original overpowering fighting style showcased in this fight, and Denji's personal, more tactical fighting style drilled into him by Kishibe. The only thing that wasn't explicitly set up is the ability to survive without the heart by using Power's blood, but we know she's a Blood Devil so it won't be a stretch to believe she can do that much for a short while as part of her contract.
Furthermore, this resolution to their conflict is consistent with multiple themes permeating the entire narrative:
* the Aesop fable about the country mouse and the town mouse (Makima chose to be the town mouse at the risk of being eaten);
* Denji's desire to eat good food, particularly lots of steak;
* appreciating the intimacy through knowledge of the partner ('s taste) and Makima's own failure to recognize Pochita as the real Chainsaw Man;
* Makima's ironic comeuppance in being eaten not by her hero Chainsawman whom she idolized but by Denji whom she hated.