When I say "falling" and "spiking" I am referring to chart position. The raw numbers put this FE at 221k so far. Fates sold about 100k more during it's first week alone in Japan. At first glance it seems like Fire Emblem has lost some of its base in Japan. I am wondering if they are actually just waiting for the Lite edition in order to get Fire Emblem.
As others have mentioned, there will drop off going from 3DS to Switch for some franchises.
That and Fates has lost a lot of its goodwill and isn't perceived as one of the better entries in the series by some fans.
And also, the Oninaki comments were so harsh.