In general, the idea of drama and a rival in the game is a good idea. Many other games have implemented it successfully. But Eric was not a good obstacle for an incest game. Maybe he's exactly what's expected in an NTR game, but for incest fans he was the exact opposite of what they wanted. He's not so much an obstacle as a brick wall. I've played against many a rival in other incest games, but never faced an opponent as unnecessarily hard as Eric. As has been said, the "reward" in defeating Eric is just to keep him away from some of your family members. For NTR fans, the reward of seeing him screw your family is torture to the incest fans.
I'm definitely not opposed to a male rival in a future game from DS. But assuming the game is billed as incest and not NTR, that rival should be more easily defeated or foiled. Not easily, but not impossibly hard, either. They can also screw things up for you without ending the game, or having sex with too many of your family members. They could put something in there for the NTR fans where maybe you help them screw your family in exchange for something, but that would be a choice on your part.
But Eric is a lost cause for most people in this game. There are too many bad memories of him, and the NTR fans keep bringing him up again and again, thus starting the conflict over. Each argument reminds DS of the shitstorm he'll face if he announces an Eric-heavy update, thus making it less likely. As far as most incest fans are concerned, there's no way to redeem Eric at this point, or to put more of him in the game without antagonizing people.