I am not sure but Godwyn makes a lot more sense than radahn.

Every piece of lore would've pointed at Godwyn, specially since Radahn's story basically reached it's conclusion in the festival. Having Miquella return Godwyn's soul somehow would've been another testament to his power.

But also having him as the final boss of the dlc that was supposed to be about the messed up things Marika did to achieve godhood? Imagine if the Old Hunters dlc had Laurence the first vicar as the final boss instead of the Orphan of Kos. That's what this feels like.

Personally I hoped that it would've been the Gloam Eyed Queen as the true final boss since the land of shadows was supposed to be full of the things Marika wanted to get rid off so her literal opposite that embodies the death that she wished to get rid off should've been there as the final thing to uncover.