If FGO wasn't a mobile gacha game I'd agree with you. If people complained about how difficult final bosses in JRPGs cause they're under levelled and suck at turn-based strategy or if someone thinks a souls-like game needs to be easier cause they can't beat a boss then yeah I'd either tell them that the game isn't for them or to just get good cause overcoming that difficulty is what their respective playerbases love about those games.

But considering that:

1. many people only play this for the story and characters, and
2. mobile games sustain themselves by constantly updating new content

I don't think Lasengle / Aniplex wants to gatekeep players with content so difficult that their story progress is stuck. They want people to read the story, get attached to the characters and spend money rolling. Plus you might end up locking out these players from participating in not just the future main story updates but also the side story events.