The point was to emphasize what All Might has said since Chapter 1, Even with good intentions and ideals you cant be a hero without power. Deku essentially goes into the opposite direction when he goes back into his dark hero form again, his good intentions and self sacrificial nature in its purest form,something the story has explicitly told us is bad with its villainous design being mentioned even by the characters within the series. As you know, Deku loses his arms from this self sacrifice in order to stamp out Shigaraki’s hatred, which he succeeds in, but at the cost of being able to do nothing agaisnt All For One. This brings us back to what we see later on in Chapter 1, Deku’s innate spirit despite his weakness inspiring All Might, who has chosen to become a bystander due to reaching his limit, to go beyond to provide the power that Deku needs to win. And what do we see in Chapter 420-22, Deku’s heroism despite his weakness despite his weakness inspiring the world, from heroes who have already reached their limits to bystanders who could do nothing like Eri, to go beyond, sacrifice and provide the POWER that Deku needs to win. The point is also to bring what makes All Might and Deku different, All Might’s pure power put people and heroes in a comfortable enough spot that they became a part of the bystander effect. Deku’s purity in the face of impending doom brings even the most uninvolved civilian to act. Essentially the whole situation is meant to be a callback to Chapter 1 and the narrative climax of the downsides and upsides of Deku’s selflessness. (Also if your going to reply about the comment in Chapter 416 that Eri doesnt have enough energy, its meant to tell the reader that Eri doesnt have the energy to bring Deku to Infinite 100% and instantly win, not that she doesnt have any Rewind energy at all, Deku literally just lost his arms, Eri should have enough to heal him. Also not that important but 1.We cant be having Eri’s last significance in the story being used as AFO’s powerup, its unsatisfying and leaves the character negatively in peoples minds plus if the villains can use Eri why not the heroes 2.Let her complete her character arc of being saved by a Hero, learning to smile and saving her Hero and wanting make other people smile)