As much as I understand where you are coming from, the game is only 18+ because the creator felt some of the things shown were too heavy for a normal audience but, if the game were properly assessed, she feels it would end up being categorised as PG 15. Moreover, I don't see it as a reason to feel bad about your fetish (seeing as I also share in it) but rather as a reminder that it should be kept safe, and a show of what the consequences can be for all involved. It is, to echo some of the arguments above, a refreshing take on the genre that focuses deeply on the other side of the argument regarding hypnosis/mind control, that being the life the player will be ruining. Again, my views are completely opinion-based and I am fine with them being challenged; I feel that is, in part, a large aspect of the game; being able to see things through a different lens.
Of course, this is my disclaimer that I am by no means an impartial commentor; I am heavily into the genre, but I also adore how well-written the story is (Penlight is still my favourite VN to date), and I admire the creator greatly from the point of view of me being an amateur writer. Also, I just want to say that I didn't feel judged playing /reading through the game, but rather questioned as to whether I fully understood what I liked, and that is an invaluable contribution to my experience having realised I was into this and what it entailed.
As for the target audience, it really could be anyone who appreciates good writing or is into the genre, but will of course resonate most greatly with those that enjoy both. I've also heard good things from people to whom I recommended the VN on the basis of a morality story, rather than a hypnosis one, so there is that to consider as well.
To end the rant to a 1 year old post, I'd just like to say that consent is important, even if some people may find themselves more enticed by a lack of it, that can be left squarely within the realm of roleplay, so I am both grateful and happy about the fact that all routes that violate this sense of consent and choice are either (objectively) bad routes, or routes that require redemption to achieve a good ending.