Not sure if this got brought up earlier in the thread but I couldn't find it. I sort of have to question the longevity of this game in the longterm from a design standpoint. I'm going to make comparisons to two very popular mobile titles I think Japan tends to be familiar with: Granblue Fantasy and Fate/Grand Order. When it comes to longevity I think goals are really important. What I mean by that is that the game's design is easy but complex enough to allow for progression in the game to feel like something meaningful and has a level of depth that allows for customization.
Let's use the two aforementioned games as an example. Progression in Granblue is inifinitely more complex than it is in F/GO. For one, GBF's progression depends on a large character pool, building on multiple elements, working towards weapon grids consistently. Between these three pinnacles, GBF has long legs and progression is something that never ever ends. Even within a single element there are a variety of weapon pools to create certain party types. Working towards progression is hard and arduous but there is a level of complexity due to the underlying design and mechanics in the game.
Then we come to F/GO. This game is not complex by any means and progression is straight simple. Level up servants and level up skills. The avenues of doing so is easy and getting to the end goal is easy as well which means the game doesn't really depend on you working really hard towards progressing to end game. You're just... doing stuff. Why is F/GO popular then? Waifubait and the fanbase. That's it. While people play GBF for the gameplay and progression, the audience of F/GO is the exact opposite. People aren't playing for progression and gameplay. They're playing for the characters.
This brings me to my fear for Heroes. I think this game falls more under the F/GO category than GBF. The progression is way too simple and the customization even more so. You level up your characters. You acquire skills. Then you're done. So what? I guess you can consider team compositions a level of complexity but let's be real, it's really not. Eh... Not sure what IS can do to sort of change this to be honest.... It'd be one thing if different weapons allowed for different pros and cons a la awakening and fates but... too late maybe?
FGO's core strength is the story. Late game chapters are fantastic, and made everything worth it. Having gone through it all though I do admit I'm burnt out on the game... I mean, how the hell do you plan on one-upping that? At best I'm expecting the kind of letdown going from UBW to Heaven's Feel on the original Fate/SN. I'm collecting dailies, going through the revived event quests and mostly sitting on a full stamina bar while I see what they plan to do next. Lack of competitive MP means I don't need to force myself to upgrade this and that.
Granblue's strength is the core gameplay. Its downfall is excess grinding and guild events and MP bosses that require way too much coordination outside of the game and way too much commitment in-game to be successful. The story has always been trash and a waste of a great cast of voice actors. I log in occasionally when there's free pulls but that's about it.
Over the long term I feel FGO's way will prevail. FEH's arena point system is great for casual play since it's not completely about who can keep their phone glued to their face all day long. There needs to be a tweak to the number of arena tokens you get daily, but otherwise there's a foundation for a good story mode complemented by a competitive mode that doesn't turn into something you absolutely loathe.
Also, considering how much of a trainwreck FGO was when it first came out IS is in a great spot w/ FEH. Yes, there will be a fair number of people quitting over the next few weeks, but refinements can and will be made if they at all care about their IP.