Ok, the magic decline actually seems reasonable now. Before this chapter, it seemed like magic declined for no other reason than "everything was peaceful, why fight?" which is nonsense. Times of peace are when one prepares for the next war.
This chapter seems to indicate most of the capable mages died in the hundred years war, and then records were intentionally burned by a dedicated group——basically a form of "brain drain", and something I can totally see happening.
Unfortunately, the story never hinted this might have been the case before the last couple chapters. It kinda feels like the author read complaints about his bad worldbuilding, and threw something in at random.