FE fans are hardcore. Buy all the DLC. LEs sell out instantly. Mobile game is the core FE experience without all the extra marriage/support stuff to distract you (kind of like Valentia).

On the Gacha mechanic if you can restrain yourself and deal with not having everything, you can totally play the game and do really well without spending a dime. (It is a strategy game, after all, so figuring out how to best a stacked against you situation is part and parcel for the series.) The bolded part is a big IF, though, and that's why it's pulling in a lot of money pretty consistently.