Another aspect I mentioned in the anime room
It is quite easier to make a card needed than it is to make a character needed in a 3d game.
In a "card" game, all you have to do is make up a debuff, a gimmick that only that card can resolve
After all, think Magic... can you clip thru a wall, abuse enemy AI or use some other creative exploit to beat the "boss" with 0 effort?
Even the "difficult" ones, Demon / Dark Soul...
Plenty of "Beat boss with zero effort" exploits and videos.
So, you will have to either be VERY blatant.... like only that character can even DAMAGE the boss, or players will find a way.
So, forget about "walling off" Free players.
Last issue that comes to mind now, will add more as soon as I remember.
I watched some gameplay videos
Take a quest like this one.
(not the video I watched, so please forgive me if the player starts using bad words for example)
This quest has you searching for a treasure, it is interesting because you get actual clues and have to find out their meaning. Nothing too complex, but it is not your average "kill # slimes" or "collect # apples" thing.
At the end of the quest you go to a dungeon and face a boss. Before the boss fight you are joined by the character that sent you on the hunt.
Problem: technically you can get that character from gatcha, so he could be in your party.
The STORY is written so that he is NOT with you during the event, but you can still add him to the party even if it makes no sense.
In a normal JRPG they would just either have him join you AFTER the quest or temporarily remove him DURING the quest.
Genshin obviously can't do either.
The gatcha even hurts the story.