Well, you may reject it, but that doesn't make it anymore true in my opinion. The game was never meant to be like DMD, BigBrother and others where it's basically just about the incest. I mean, alone the fact that you won't be able to date neither Diane nor Jenny or take them to prom shows that. It was always meant to be a virtual novel/dating sim, where you can basically fuck the whole town (which obviously would include relatives) and that's how it has been advertised from day one.
What you are describing are the necessary developments for a boy who still goes to school and who still lives with his familie, loses his dad, etc. but it never was meant to be "the end game". Yes, Incest is a part of the story but it's not what the story is about. It has never been like for example SnowDaze where the player knew he'd have to conquer his house and that's it.
Or to put if differently, the game could have been in the current, slightly adapted setting from the start and things would have been fine. Of course, incest was a major fetish at the time and every devs wants to include it (especially when it's in a school setting) but that doesn't make every game which has incest somewhere "about" incest. As said, DMD for example doesn't make any sense without the incest, which is the complete opposite of SummertimeSaga.
Else it's not true that other characters will have smaller stories. Roxxy's story for example is quite a bit bigger than Diane's and the other schoolgirls will follow, especially once the dating system actually works and MC can go to dates and start his harem in the beach house. There are basically 9 main girls in the game who will have more or less equal amount of content once it's finished. We've got Debbie, Jenny, Diane as family and Roxxy, Eve, Mia, Ronda, Annie and Judith as other girls. Else there are some intermediate characters like June, Mrs. Johnson, Helen and the teachers, etc. who'll get more content than just one scene but not as much as the main cast. To me that's a vast range which dwarfs the incest, especially since one will actually be able to finish the game without having sex with family members (at least as things stand now).
Well, that's partly for "technical"reasons. Their stories where the first to start, their stories will be the first to finish (I mean, you can't introcude pregnancy until a character's story is done,right?) and their stories have to be reworked now because the code base has to be changed and because they cause bugs and problems (like the odd reliance on Jenny/Somrak for Dex,which will be changed). I mean, it's not like DC gaves us any options, so it wasn't really a public vote, but a poll solely between Diane and Jenny. That said, I still think they would have won somewhat close, simply because people always liked to conitue characters who already had some content and yes, because there are people who like incest.