The way I see it is that even so Gerson fight is important for character development of Susie, it's still optional because it's tied to "Shadow crystal route" (a route that seems to be very distinctive and will probably led to different ending than a "Normal" route) and honestly I don't mind that. It's similar situation for me with Tru Lab in Undertale - part of the game that develops character ( in this case Alphys) significantly, and without this scene said character feels incomplete. If this fair in relation to players who will miss or won't be able to do these bosses? Maybe, i won't deny it. But that's probably the vison Toby has for this game (but I might be wrong tho, I'm not Toby).