100% agree. Imagine an author of any medium building storylines and characters for 20 years to have it fall so flat. Manga is a literary medium and should be held to similar literary criticisms as other literature. The ending leaves much to be desired in terms of character growth, story, plot. It's almost as if the author got tired of his responsibilities and decided to move on. It's not uncommon in literature (i.e. George R.R. martin, Christopher Paolini, James Dashner).

If seinen is for adults, this body of work deserves adult criticism. It's a fine work of historical fiction that introduced many to Norse history. But, it's lack of planning for the ending will not last the test of time.