As an F:GO player, I imagine the story can go about the same way it does for us.
You do realize that FGO is a game and not a written story like this fanfic is? Plus, this is also a serial fanfic posted in a forum. So the author is able to get feedback from the readers, as well as reader requests. Both good and bad reader feedback and requests.
For FGO the game, they know that it is a gatcha game (which means they don't know what servants you have) plus, what you are doing is gameplay related and not story related. You are picking characters that is affecting your gameplay.
But this is a story. And people will wonder, what is all these 300+ characters doing. They can't just be sitting around in the background doing nothing. You have a small towns worth of servants and it makes no sense that they are doing absolutely nothing. Not with all of their strong personalities. Within a game, the story writers and management can and will decide on how the story will proceed, even if it doesn't make sense to the story of the game.
And then there are the fans, the people that are asking about what is their favorite character doing? OMG, I love the interaction between this character and this other character, I want to see more! Where is my favorite character, why are you ignoring my favorite character! Why are you making this servant act this way because they will never act this way at all! And this is just a small example of the BS that can get put in by fans with strong emotions.
This is why I brought this up. Because when you have too many characters and you get demands to give each and every single character equal screentime, the story gets bogged down by minutiae and the author is stuck writing down all about these character interactions and not about the story itself. At least for FGO, you have the excuse of the singularities. So each singularity helps to limit the number of characters he needs to write but that also means the chapters that happen outside of Chaldea might end up being just as long as the singularities due to the shear number of servants that have to be written.
But honestly, considering just how much time/story each singularity is taking in written format, I sincerely wonder how long the author can keep writing this story before burning out and giving up. I mean, seriously, 50 chapters based on the threadmark, and we're only into the second singularity. And this is only posted one chapter per week, so this is almost hitting up to a year. And we are still in the second singularity.
At this rate, this will take another 5 years just for the first story arc to complete. At any rate, I'll continue to enjoy the story as it is.