FGO took like 8 months or so to blow up to the popularity it has now.
And a lot of that is thanks to the event variety, limited gacha diversity, new gacha illustrations, and crossover fan service.
It's by no means a particularly good or deep smartphone RPG compared to say Granblue, but the amount of advertising it got both paid and free twitter advertisement helped it bounce back a lot and become one of the stronger performing games on smartphones over the past few months.
Like for a couple months after the whole Scathach incident, they really gave me no real reason to play the game until they introduced their new event series like Saber Wars or Garden of Sinners collab.
"Whales" and big spenders make a ton of money for the smartphone game, and while that is nice, if you don't provide content that keeps everyone from free to play to spenders interested, spenders/"whales" are going to move onto a different game and it will die because of a dying userbase.
Fire Emblem has the benefit of a lot of production value behind it so far with the voice acting and the illustrations, but it's a big investment that they're going to need to keep building on for the game itself to be successful. And that means keeping these voice actors on contract for story events and continually writing new scenarios and maps to give players a reason to keep playing.
It's really not so much about how big a property is, but rather how much the fans within that property are willing to spend. For example Fate might not sell as much as Fire Emblem, which means the reach of the franchise is more narrow, but that doesn't mean that Fire Emblem fans are more likely to want to spend a lot on a mobile game. Meanwhile, Fate fans have already been well trained to overspend on overpriced products, since we know the anime sells well in Japan despite being released on box sets which cost hundreds of dollars.
I see. Very curious to see how this performs out of the gate and where it is later in the year.