You just all need to take into consideration one thing, one thing only, a thing that devs care only about in this game, at least based on what they were telling me multiple times on discord.

The only thing they care about is core players. Those core players give no fucks about any game challenges, they only care about their harem. Thus it doesn't matter to them if they one shot everything in this game, as long as they could work on harem they would. So they could farm trolls to get girls? They would. They had some affection scenes to unlock, they would buy gifts and unlock them. Apart from that? Nothing matters. So if they have all girls at max affection/level, they would not do nearly anything in game, and thats what devs don't want.

So don't think they will reduce the costs, because that would mean making upgrading affection to easy, and thus make less work for core players. And less work mean less game time, as devs think this game is some kind of odd clicker game.

As a side note: as someone already explained, you get less cash per collect from harem, but you get collects more often, so cash/h actually gone up, cash per collecting once all girls are ready to collect gone down.

As a side note 2: if you talk a bit with devs, you will get a feeling they are kinda full of themselves, whatever they fuck up, they will tell you it wasn't a fuck up, but a change necessary to accommodate future changes to make game better, etc. Whatever people complain about that they changed, they will repair it in convoluted way, to like make you think they are fixing the problem while you could argue they don't (like this whole affection thing).

As a side note 3: I think this game requires the most work or real money spent on kobans to unlock girls/their affection scenes, and this cost now goes up with each new girl, the few single image scenes that often aren't even worth calling h-content are all you really get, there is no voice acting of any sort, no proper h-scenes/story, you don't really learn about girls, its hard to call this a wifu collecting game. It actually makes you wonder how the hell this game survived so long on the market, thou I will admit there are worse games out there.