Sure, it's a somewhat important part of the game, but it's not what the game is about. We aren't talking about games like DMD or let's say Light of my life, where the incest is basically the main driver of the game and where the game makes way less sense without it. In these cases I could totally understand that people would be upset, if the incest was to be taken lightly. In SS's case on the other hand it's just one fetish among many.
I think these points excatly show why SS isn't an incest game. Even back in the days when there was real incest, everything was pretty superficial at best. There never was an elaborated taboo angle which went beyond a few basic plot points. Jenny and Diane never really gave a fuck in the first place and in Debbie's case the resistance is just about as big as it had to be, to be credible, where she essentially gives in almost right away and just goes with it after that, as does everybody else. Yes, it ticks important boxes here and there, but that's more or less it?
Edit: It's by the way just the same as with most other fetishes. SS is good at including a lot of them, but neither is actually really elaborated. You like pregnant stuff? Sure, but you'll only get a few sex scenes and no real connecting story content. You like trans content? Yes, there's Eve but she won't fuck MC. You like hotwifing? Yes, there's Tony, but it's the most basic plot imaginable. You like BDSM? Yes, there's a nun who is going to spank someone a few times, but that's it. And so on...