Niche? It stopped being after Blazing Sword, which sold around one million; the problem was that the franchise entered a decline in sales, but it happens to most franchises (it didn't help that finding a Radiant Dawn copy was hard as heck). They revitalized it with Awakening, saving the franchise indeed, but Awakening isn't what made the franchise be mainstream. It just popularized its potential.
And SRPGs overall are niche? There's a lot of unsuccessful or obscure SPRGs, but there are also a lot of successful ones.
Regarding your question, the success of the mobile game doesn't equal to the success of the franchise on handhelds and home consoles, though. The mobile game could be a success and the subsequent entry could fail miserably. The mobile market is way more volatile than the other, although a successful entry there can give hope for more games (on handhelds and home consoles).
For starters, they could not butcher the next sequel into two different parts and sell the third option as DLC. This ain't pokemon. The amount of marketing and directs given to these games was great, so they should keep it that way. Things like Corrin on Smash were also helpful, so I guess the next FE Lord in Mario Kart 9?