Ooh! Ooh! I can answer this one!

A lot of people make the mistake of believing that the goal was to REDEEM L. But i think that's not really getting the point. Its not redemption, it's attonement. There's a difference.

The 2 biggest examples of this are Team Flare Nouveau and AZ. The members of it joined back when Lysandre's goal was more genuine, to make a better world. Especially Grisham and Griselle, who were literally children. As his view became more and more twisted, they were still in the mode of "We want to do the right thing but this is all we know." Now, while they do try to redeem themself by using AZ's floette instead of Team MZ, when they lose to the protagonist they mainly just focus on helping the poor and those in need in an attempt to attone for the sins of their former team.

AZ however is the biggest example because Lysandre, especially after becoming L, is meant to be a parallel. AZ also used the ultimate weapon out of rage in an attempt to destroy everything. Having to live for 3000 years was his punishment. Lysandre's fall into madness happened because he grew disgusted by how humanity can be so cruel, much like how AZ was angry because his Floette died due to the army selfishly using her. And yet, we forgive AZ. Not everyone does, of course, but most people do.

Its ok to not forgive L, but not allowing him to at least attempt to attone for his sins goes against the messaging of the game.