If Susan wasn't in the game, you'd probably never end up with Rachel. Or at least the relationship would be completely different.
Susan is your mentor when it comes to BDSM and not only shares Rachel but basically pushes the MC.
All starting with the sessions Susan and Rachel have where MC is allowed to participate. Rachel is pretty pissed in the beginning.
The game would be better without Susan? I don't think so.
MC wouldn't develop the way he did without Susan's guidance.
Yeah I agree with this. I like Susan's purpose in the plot, having Susan, an agent of Rachel's husband, already be controlling her makes it much easier for her to accept a second male agent sent by her husband coming into her life and also controlling her. Having Susan already there makes it easier to slot in the MC as a male Susan without needing for the family to come to terms with the idea of being dominated by someone because they're already used to being dominated by someone. Susan makes the entire plot more believable and immersive and I think that's a strength of the writing in this series.
That being said, I can imagine alternative solutions keeping in the same plot, Susan can be for example retired or away, and Rachel is being controlled by her or her memory remotely. Susan can also not be a character but a set of guidelines she abides by, maybe negotiated with her husband or family tradition or something like that. But I imagine the author enjoys femdom so that's why he put in Susan as is, and it's fine since it's disable-able.
The other 2 solutions that I see more commonly are to make Rachel very submissive and wanting to experiment with submission of her own volition or to make the MC make some Amy-esque Machiavellian moves to take control, but those are totally different plots that are more tropey. I think I've seen them more in other games, so I appreciate how this game's plot line goes currently. It just feels more natural and believable without hard-to-empathize-with tropes like these.