But at some point you are also gonna max out a girl's devotion. So at some point you will have to stop using a girl and play with the others.
Again, why? If someone likes to play the scenario mainly with two or three girls because he likes their playstyle, then he should be able to.
I just think the devs should determin a general direction. If they actually tie the girls more into the story etc. (for example by unlocking some scenes or the devotion dialogs etc. that way) then it's totally fine to have players play them without alternative.
But since that's not the case, they also shouldn't force players to play certain girls to essentially advance the story. Just let players play whom they want and give them the option to gain the points for everybody else somewhere else. Sure, it should be more effort for the player who does it that way timewise, but it should still be doable. It's zero additional work for the devs and it'll likely solve most people's problem.
Now they are going for some kind of pretty complicated compromise for - I'd say - no real reason.
The levels already have difficulty classifications by stars, and the devs have already stated they're looking to introduce actual difficulty levels, just for folks like you that have issues using waifus in certain scenarios.
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I'm not sure if they'll really fix the problem that way. After all the girls are different and will stay different. Space Combat with Ela is easy as hell, while space combat with some others can simply be annoying at times. It's not that it's hard to do either, it's just little fun in comparison and that's what playing the game should be about?
As said, it's just not the best design when a mission is (very) easily doable with one girl, but the player is essentially forced to play another girl and "has to" then lower the difficulty for that mission to have his fun.